Clarify directory and ORPort checking functions
The OR and dir checking functions in router.c are somewhat confusing. We could document them better, or modify their names, or restructure.
In #18616 (moved), we added:
- decide_to_advertise_begindir
We already had:
- check_whether_orport_reachable
- check_whether_dirport_reachable
- router_has_bandwidth_to_be_dirserver
- router_should_be_directory_server
- dir_server_mode
- decide_to_advertise_dirport
- consider_testing_reachability