For everything we declare that starts with _, make it end with _ instead.

This is a machine-generated patch.  To make it, start by getting the
list of reserved identifiers using:

     git ls-tree -r --name-only HEAD | grep  '\.[ch]$' | \
       xargs ctags --c-kinds=defglmpstuvx -o - | grep '^_' | \
       cut -f 1 | sort| uniq

You might need gnu ctags.

Then pipe the output through this script:
==============================

use strict;

BEGIN { print "#!/usr/bin/perl -w -i -p\n\n"; }

chomp;

next if (
     /^__attribute__/ or
     /^__func__/ or
     /^_FILE_OFFSET_BITS/ or
     /^_FORTIFY_SOURCE/ or
     /^_GNU_SOURCE/ or
     /^_WIN32/ or
     /^_DARWIN_UNLIMITED/ or
     /^_FILE_OFFSET_BITS/ or
     /^_LARGEFILE64_SOURCE/ or
     /^_LFS64_LARGEFILE/ or
     /^__cdecl/ or
     /^__attribute__/ or
     /^__func__/ or
    /^_WIN32_WINNT/);

my $ident = $_;

my $better = $ident;
$better =~ s/^_//;

$better = "${better}_";

print "s/(?<![A-Za-z0-9_])$ident(?![A-Za-z0-9_])/$better/g;\n";
==============================

Then run the resulting script on all the files you want to change.
(That is, all the C except that in src/ext.)  The resulting script was:

==============================

s/(?<![A-Za-z0-9_])_address(?![A-Za-z0-9_])/address_/g;
s/(?<![A-Za-z0-9_])_aes_fill_buf(?![A-Za-z0-9_])/aes_fill_buf_/g;
s/(?<![A-Za-z0-9_])_AllowInvalid(?![A-Za-z0-9_])/AllowInvalid_/g;
s/(?<![A-Za-z0-9_])_AP_CONN_STATE_MAX(?![A-Za-z0-9_])/AP_CONN_STATE_MAX_/g;
s/(?<![A-Za-z0-9_])_AP_CONN_STATE_MIN(?![A-Za-z0-9_])/AP_CONN_STATE_MIN_/g;
s/(?<![A-Za-z0-9_])_assert_cache_ok(?![A-Za-z0-9_])/assert_cache_ok_/g;
s/(?<![A-Za-z0-9_])_A_UNKNOWN(?![A-Za-z0-9_])/A_UNKNOWN_/g;
s/(?<![A-Za-z0-9_])_base(?![A-Za-z0-9_])/base_/g;
s/(?<![A-Za-z0-9_])_BridgePassword_AuthDigest(?![A-Za-z0-9_])/BridgePassword_AuthDigest_/g;
s/(?<![A-Za-z0-9_])_buffer_stats_compare_entries(?![A-Za-z0-9_])/buffer_stats_compare_entries_/g;
s/(?<![A-Za-z0-9_])_chan_circid_entries_eq(?![A-Za-z0-9_])/chan_circid_entries_eq_/g;
s/(?<![A-Za-z0-9_])_chan_circid_entry_hash(?![A-Za-z0-9_])/chan_circid_entry_hash_/g;
s/(?<![A-Za-z0-9_])_check_no_tls_errors(?![A-Za-z0-9_])/check_no_tls_errors_/g;
s/(?<![A-Za-z0-9_])_c_hist_compare(?![A-Za-z0-9_])/c_hist_compare_/g;
s/(?<![A-Za-z0-9_])_circ(?![A-Za-z0-9_])/circ_/g;
s/(?<![A-Za-z0-9_])_circuit_get_global_list(?![A-Za-z0-9_])/circuit_get_global_list_/g;
s/(?<![A-Za-z0-9_])_circuit_mark_for_close(?![A-Za-z0-9_])/circuit_mark_for_close_/g;
s/(?<![A-Za-z0-9_])_CIRCUIT_PURPOSE_C_MAX(?![A-Za-z0-9_])/CIRCUIT_PURPOSE_C_MAX_/g;
s/(?<![A-Za-z0-9_])_CIRCUIT_PURPOSE_MAX(?![A-Za-z0-9_])/CIRCUIT_PURPOSE_MAX_/g;
s/(?<![A-Za-z0-9_])_CIRCUIT_PURPOSE_MIN(?![A-Za-z0-9_])/CIRCUIT_PURPOSE_MIN_/g;
s/(?<![A-Za-z0-9_])_CIRCUIT_PURPOSE_OR_MAX(?![A-Za-z0-9_])/CIRCUIT_PURPOSE_OR_MAX_/g;
s/(?<![A-Za-z0-9_])_CIRCUIT_PURPOSE_OR_MIN(?![A-Za-z0-9_])/CIRCUIT_PURPOSE_OR_MIN_/g;
s/(?<![A-Za-z0-9_])_cmp_int_strings(?![A-Za-z0-9_])/cmp_int_strings_/g;
s/(?<![A-Za-z0-9_])_compare_cached_resolves_by_expiry(?![A-Za-z0-9_])/compare_cached_resolves_by_expiry_/g;
s/(?<![A-Za-z0-9_])_compare_digests(?![A-Za-z0-9_])/compare_digests_/g;
s/(?<![A-Za-z0-9_])_compare_digests256(?![A-Za-z0-9_])/compare_digests256_/g;
s/(?<![A-Za-z0-9_])_compare_dir_src_ents_by_authority_id(?![A-Za-z0-9_])/compare_dir_src_ents_by_authority_id_/g;
s/(?<![A-Za-z0-9_])_compare_duration_idx(?![A-Za-z0-9_])/compare_duration_idx_/g;
s/(?<![A-Za-z0-9_])_compare_int(?![A-Za-z0-9_])/compare_int_/g;
s/(?<![A-Za-z0-9_])_compare_networkstatus_v2_published_on(?![A-Za-z0-9_])/compare_networkstatus_v2_published_on_/g;
s/(?<![A-Za-z0-9_])_compare_old_routers_by_identity(?![A-Za-z0-9_])/compare_old_routers_by_identity_/g;
s/(?<![A-Za-z0-9_])_compare_orports(?![A-Za-z0-9_])/compare_orports_/g;
s/(?<![A-Za-z0-9_])_compare_pairs(?![A-Za-z0-9_])/compare_pairs_/g;
s/(?<![A-Za-z0-9_])_compare_routerinfo_by_id_digest(?![A-Za-z0-9_])/compare_routerinfo_by_id_digest_/g;
s/(?<![A-Za-z0-9_])_compare_routerinfo_by_ip_and_bw(?![A-Za-z0-9_])/compare_routerinfo_by_ip_and_bw_/g;
s/(?<![A-Za-z0-9_])_compare_signed_descriptors_by_age(?![A-Za-z0-9_])/compare_signed_descriptors_by_age_/g;
s/(?<![A-Za-z0-9_])_compare_string_ptrs(?![A-Za-z0-9_])/compare_string_ptrs_/g;
s/(?<![A-Za-z0-9_])_compare_strings_for_pqueue(?![A-Za-z0-9_])/compare_strings_for_pqueue_/g;
s/(?<![A-Za-z0-9_])_compare_strs(?![A-Za-z0-9_])/compare_strs_/g;
s/(?<![A-Za-z0-9_])_compare_tor_version_str_ptr(?![A-Za-z0-9_])/compare_tor_version_str_ptr_/g;
s/(?<![A-Za-z0-9_])_compare_vote_rs(?![A-Za-z0-9_])/compare_vote_rs_/g;
s/(?<![A-Za-z0-9_])_compare_votes_by_authority_id(?![A-Za-z0-9_])/compare_votes_by_authority_id_/g;
s/(?<![A-Za-z0-9_])_compare_without_first_ch(?![A-Za-z0-9_])/compare_without_first_ch_/g;
s/(?<![A-Za-z0-9_])_connection_free(?![A-Za-z0-9_])/connection_free_/g;
s/(?<![A-Za-z0-9_])_connection_mark_and_flush(?![A-Za-z0-9_])/connection_mark_and_flush_/g;
s/(?<![A-Za-z0-9_])_connection_mark_for_close(?![A-Za-z0-9_])/connection_mark_for_close_/g;
s/(?<![A-Za-z0-9_])_connection_mark_unattached_ap(?![A-Za-z0-9_])/connection_mark_unattached_ap_/g;
s/(?<![A-Za-z0-9_])_connection_write_to_buf_impl(?![A-Za-z0-9_])/connection_write_to_buf_impl_/g;
s/(?<![A-Za-z0-9_])_ConnLimit(?![A-Za-z0-9_])/ConnLimit_/g;
s/(?<![A-Za-z0-9_])_CONN_TYPE_MAX(?![A-Za-z0-9_])/CONN_TYPE_MAX_/g;
s/(?<![A-Za-z0-9_])_CONN_TYPE_MIN(?![A-Za-z0-9_])/CONN_TYPE_MIN_/g;
s/(?<![A-Za-z0-9_])_CONTROL_CONN_STATE_MAX(?![A-Za-z0-9_])/CONTROL_CONN_STATE_MAX_/g;
s/(?<![A-Za-z0-9_])_CONTROL_CONN_STATE_MIN(?![A-Za-z0-9_])/CONTROL_CONN_STATE_MIN_/g;
s/(?<![A-Za-z0-9_])_CPUWORKER_STATE_MAX(?![A-Za-z0-9_])/CPUWORKER_STATE_MAX_/g;
s/(?<![A-Za-z0-9_])_CPUWORKER_STATE_MIN(?![A-Za-z0-9_])/CPUWORKER_STATE_MIN_/g;
s/(?<![A-Za-z0-9_])_crypto_dh_get_dh(?![A-Za-z0-9_])/crypto_dh_get_dh_/g;
s/(?<![A-Za-z0-9_])_crypto_global_initialized(?![A-Za-z0-9_])/crypto_global_initialized_/g;
s/(?<![A-Za-z0-9_])_crypto_new_pk_from_rsa(?![A-Za-z0-9_])/crypto_new_pk_from_rsa_/g;
s/(?<![A-Za-z0-9_])_crypto_pk_get_evp_pkey(?![A-Za-z0-9_])/crypto_pk_get_evp_pkey_/g;
s/(?<![A-Za-z0-9_])_crypto_pk_get_rsa(?![A-Za-z0-9_])/crypto_pk_get_rsa_/g;
s/(?<![A-Za-z0-9_])_DIR_CONN_STATE_MAX(?![A-Za-z0-9_])/DIR_CONN_STATE_MAX_/g;
s/(?<![A-Za-z0-9_])_DIR_CONN_STATE_MIN(?![A-Za-z0-9_])/DIR_CONN_STATE_MIN_/g;
s/(?<![A-Za-z0-9_])_DIR_PURPOSE_MAX(?![A-Za-z0-9_])/DIR_PURPOSE_MAX_/g;
s/(?<![A-Za-z0-9_])_DIR_PURPOSE_MIN(?![A-Za-z0-9_])/DIR_PURPOSE_MIN_/g;
s/(?<![A-Za-z0-9_])_dirreq_map_get(?![A-Za-z0-9_])/dirreq_map_get_/g;
s/(?<![A-Za-z0-9_])_dirreq_map_put(?![A-Za-z0-9_])/dirreq_map_put_/g;
s/(?<![A-Za-z0-9_])_dns_randfn(?![A-Za-z0-9_])/dns_randfn_/g;
s/(?<![A-Za-z0-9_])_dummy(?![A-Za-z0-9_])/dummy_/g;
s/(?<![A-Za-z0-9_])_edge(?![A-Za-z0-9_])/edge_/g;
s/(?<![A-Za-z0-9_])_END_CIRC_REASON_MAX(?![A-Za-z0-9_])/END_CIRC_REASON_MAX_/g;
s/(?<![A-Za-z0-9_])_END_CIRC_REASON_MIN(?![A-Za-z0-9_])/END_CIRC_REASON_MIN_/g;
s/(?<![A-Za-z0-9_])_EOF(?![A-Za-z0-9_])/EOF_/g;
s/(?<![A-Za-z0-9_])_ERR(?![A-Za-z0-9_])/ERR_/g;
s/(?<![A-Za-z0-9_])_escaped_val(?![A-Za-z0-9_])/escaped_val_/g;
s/(?<![A-Za-z0-9_])_evdns_log(?![A-Za-z0-9_])/evdns_log_/g;
s/(?<![A-Za-z0-9_])_evdns_nameserver_add_impl(?![A-Za-z0-9_])/evdns_nameserver_add_impl_/g;
s/(?<![A-Za-z0-9_])_EVENT_MAX(?![A-Za-z0-9_])/EVENT_MAX_/g;
s/(?<![A-Za-z0-9_])_EVENT_MIN(?![A-Za-z0-9_])/EVENT_MIN_/g;
s/(?<![A-Za-z0-9_])_ExcludeExitNodesUnion(?![A-Za-z0-9_])/ExcludeExitNodesUnion_/g;
s/(?<![A-Za-z0-9_])_EXIT_CONN_STATE_MAX(?![A-Za-z0-9_])/EXIT_CONN_STATE_MAX_/g;
s/(?<![A-Za-z0-9_])_EXIT_CONN_STATE_MIN(?![A-Za-z0-9_])/EXIT_CONN_STATE_MIN_/g;
s/(?<![A-Za-z0-9_])_EXIT_PURPOSE_MAX(?![A-Za-z0-9_])/EXIT_PURPOSE_MAX_/g;
s/(?<![A-Za-z0-9_])_EXIT_PURPOSE_MIN(?![A-Za-z0-9_])/EXIT_PURPOSE_MIN_/g;
s/(?<![A-Za-z0-9_])_extrainfo_free(?![A-Za-z0-9_])/extrainfo_free_/g;
s/(?<![A-Za-z0-9_])_find_by_keyword(?![A-Za-z0-9_])/find_by_keyword_/g;
s/(?<![A-Za-z0-9_])_free_cached_dir(?![A-Za-z0-9_])/free_cached_dir_/g;
s/(?<![A-Za-z0-9_])_free_cached_resolve(?![A-Za-z0-9_])/free_cached_resolve_/g;
s/(?<![A-Za-z0-9_])_free_duplicate_routerstatus_entry(?![A-Za-z0-9_])/free_duplicate_routerstatus_entry_/g;
s/(?<![A-Za-z0-9_])_free_link_history(?![A-Za-z0-9_])/free_link_history_/g;
s/(?<![A-Za-z0-9_])_geoip_compare_entries(?![A-Za-z0-9_])/geoip_compare_entries_/g;
s/(?<![A-Za-z0-9_])_geoip_compare_key_to_entry(?![A-Za-z0-9_])/geoip_compare_key_to_entry_/g;
s/(?<![A-Za-z0-9_])_hex_decode_digit(?![A-Za-z0-9_])/hex_decode_digit_/g;
s/(?<![A-Za-z0-9_])_idxplus1(?![A-Za-z0-9_])/idxplus1_/g;
s/(?<![A-Za-z0-9_])__libc_enable_secure(?![A-Za-z0-9_])/_libc_enable_secure_/g;
s/(?<![A-Za-z0-9_])_log_debug(?![A-Za-z0-9_])/log_debug_/g;
s/(?<![A-Za-z0-9_])_log_err(?![A-Za-z0-9_])/log_err_/g;
s/(?<![A-Za-z0-9_])_log_fn(?![A-Za-z0-9_])/log_fn_/g;
s/(?<![A-Za-z0-9_])_log_fn_function_name(?![A-Za-z0-9_])/log_fn_function_name_/g;
s/(?<![A-Za-z0-9_])_log_global_min_severity(?![A-Za-z0-9_])/log_global_min_severity_/g;
s/(?<![A-Za-z0-9_])_log_info(?![A-Za-z0-9_])/log_info_/g;
s/(?<![A-Za-z0-9_])_log_notice(?![A-Za-z0-9_])/log_notice_/g;
s/(?<![A-Za-z0-9_])_log_prefix(?![A-Za-z0-9_])/log_prefix_/g;
s/(?<![A-Za-z0-9_])_log_warn(?![A-Za-z0-9_])/log_warn_/g;
s/(?<![A-Za-z0-9_])_magic(?![A-Za-z0-9_])/magic_/g;
s/(?<![A-Za-z0-9_])_MALLOC_LOCK(?![A-Za-z0-9_])/MALLOC_LOCK_/g;
s/(?<![A-Za-z0-9_])_MALLOC_LOCK_INIT(?![A-Za-z0-9_])/MALLOC_LOCK_INIT_/g;
s/(?<![A-Za-z0-9_])_MALLOC_UNLOCK(?![A-Za-z0-9_])/MALLOC_UNLOCK_/g;
s/(?<![A-Za-z0-9_])_microdesc_eq(?![A-Za-z0-9_])/microdesc_eq_/g;
s/(?<![A-Za-z0-9_])_microdesc_hash(?![A-Za-z0-9_])/microdesc_hash_/g;
s/(?<![A-Za-z0-9_])_MIN_TOR_TLS_ERROR_VAL(?![A-Za-z0-9_])/MIN_TOR_TLS_ERROR_VAL_/g;
s/(?<![A-Za-z0-9_])_mm_free(?![A-Za-z0-9_])/mm_free_/g;
s/(?<![A-Za-z0-9_])_NIL(?![A-Za-z0-9_])/NIL_/g;
s/(?<![A-Za-z0-9_])_n_openssl_mutexes(?![A-Za-z0-9_])/n_openssl_mutexes_/g;
s/(?<![A-Za-z0-9_])_openssl_dynlock_create_cb(?![A-Za-z0-9_])/openssl_dynlock_create_cb_/g;
s/(?<![A-Za-z0-9_])_openssl_dynlock_destroy_cb(?![A-Za-z0-9_])/openssl_dynlock_destroy_cb_/g;
s/(?<![A-Za-z0-9_])_openssl_dynlock_lock_cb(?![A-Za-z0-9_])/openssl_dynlock_lock_cb_/g;
s/(?<![A-Za-z0-9_])_openssl_locking_cb(?![A-Za-z0-9_])/openssl_locking_cb_/g;
s/(?<![A-Za-z0-9_])_openssl_mutexes(?![A-Za-z0-9_])/openssl_mutexes_/g;
s/(?<![A-Za-z0-9_])_option_abbrevs(?![A-Za-z0-9_])/option_abbrevs_/g;
s/(?<![A-Za-z0-9_])_option_vars(?![A-Za-z0-9_])/option_vars_/g;
s/(?<![A-Za-z0-9_])_OR_CONN_STATE_MAX(?![A-Za-z0-9_])/OR_CONN_STATE_MAX_/g;
s/(?<![A-Za-z0-9_])_OR_CONN_STATE_MIN(?![A-Za-z0-9_])/OR_CONN_STATE_MIN_/g;
s/(?<![A-Za-z0-9_])_OutboundBindAddressIPv4(?![A-Za-z0-9_])/OutboundBindAddressIPv4_/g;
s/(?<![A-Za-z0-9_])_OutboundBindAddressIPv6(?![A-Za-z0-9_])/OutboundBindAddressIPv6_/g;
s/(?<![A-Za-z0-9_])_PDS_PREFER_TUNNELED_DIR_CONNS(?![A-Za-z0-9_])/PDS_PREFER_TUNNELED_DIR_CONNS_/g;
s/(?<![A-Za-z0-9_])_port(?![A-Za-z0-9_])/port_/g;
s/(?<![A-Za-z0-9_])__progname(?![A-Za-z0-9_])/_progname_/g;
s/(?<![A-Za-z0-9_])_PublishServerDescriptor(?![A-Za-z0-9_])/PublishServerDescriptor_/g;
s/(?<![A-Za-z0-9_])_remove_old_client_helper(?![A-Za-z0-9_])/remove_old_client_helper_/g;
s/(?<![A-Za-z0-9_])_rend_cache_entry_free(?![A-Za-z0-9_])/rend_cache_entry_free_/g;
s/(?<![A-Za-z0-9_])_routerlist_find_elt(?![A-Za-z0-9_])/routerlist_find_elt_/g;
s/(?<![A-Za-z0-9_])_SafeLogging(?![A-Za-z0-9_])/SafeLogging_/g;
s/(?<![A-Za-z0-9_])_SHORT_FILE_(?![A-Za-z0-9_])/SHORT_FILE__/g;
s/(?<![A-Za-z0-9_])_state_abbrevs(?![A-Za-z0-9_])/state_abbrevs_/g;
s/(?<![A-Za-z0-9_])_state_vars(?![A-Za-z0-9_])/state_vars_/g;
s/(?<![A-Za-z0-9_])_t(?![A-Za-z0-9_])/t_/g;
s/(?<![A-Za-z0-9_])_t32(?![A-Za-z0-9_])/t32_/g;
s/(?<![A-Za-z0-9_])_test_op_ip6(?![A-Za-z0-9_])/test_op_ip6_/g;
s/(?<![A-Za-z0-9_])_thread1_name(?![A-Za-z0-9_])/thread1_name_/g;
s/(?<![A-Za-z0-9_])_thread2_name(?![A-Za-z0-9_])/thread2_name_/g;
s/(?<![A-Za-z0-9_])_thread_test_func(?![A-Za-z0-9_])/thread_test_func_/g;
s/(?<![A-Za-z0-9_])_thread_test_mutex(?![A-Za-z0-9_])/thread_test_mutex_/g;
s/(?<![A-Za-z0-9_])_thread_test_start1(?![A-Za-z0-9_])/thread_test_start1_/g;
s/(?<![A-Za-z0-9_])_thread_test_start2(?![A-Za-z0-9_])/thread_test_start2_/g;
s/(?<![A-Za-z0-9_])_thread_test_strmap(?![A-Za-z0-9_])/thread_test_strmap_/g;
s/(?<![A-Za-z0-9_])_tor_calloc(?![A-Za-z0-9_])/tor_calloc_/g;
s/(?<![A-Za-z0-9_])_TOR_CHANNEL_INTERNAL(?![A-Za-z0-9_])/TOR_CHANNEL_INTERNAL_/g;
s/(?<![A-Za-z0-9_])_TOR_CIRCUITMUX_EWMA_C(?![A-Za-z0-9_])/TOR_CIRCUITMUX_EWMA_C_/g;
s/(?<![A-Za-z0-9_])_tor_free(?![A-Za-z0-9_])/tor_free_/g;
s/(?<![A-Za-z0-9_])_tor_malloc(?![A-Za-z0-9_])/tor_malloc_/g;
s/(?<![A-Za-z0-9_])_tor_malloc_zero(?![A-Za-z0-9_])/tor_malloc_zero_/g;
s/(?<![A-Za-z0-9_])_tor_memdup(?![A-Za-z0-9_])/tor_memdup_/g;
s/(?<![A-Za-z0-9_])_tor_realloc(?![A-Za-z0-9_])/tor_realloc_/g;
s/(?<![A-Za-z0-9_])_tor_strdup(?![A-Za-z0-9_])/tor_strdup_/g;
s/(?<![A-Za-z0-9_])_tor_strndup(?![A-Za-z0-9_])/tor_strndup_/g;
s/(?<![A-Za-z0-9_])_TOR_TLS_SYSCALL(?![A-Za-z0-9_])/TOR_TLS_SYSCALL_/g;
s/(?<![A-Za-z0-9_])_TOR_TLS_ZERORETURN(?![A-Za-z0-9_])/TOR_TLS_ZERORETURN_/g;
s/(?<![A-Za-z0-9_])__USE_ISOC99(?![A-Za-z0-9_])/_USE_ISOC99_/g;
s/(?<![A-Za-z0-9_])_UsingTestNetworkDefaults(?![A-Za-z0-9_])/UsingTestNetworkDefaults_/g;
s/(?<![A-Za-z0-9_])_val(?![A-Za-z0-9_])/val_/g;
s/(?<![A-Za-z0-9_])_void_for_alignment(?![A-Za-z0-9_])/void_for_alignment_/g;

==============================