Closes #20435

Signed-off-by: David Goulet <dgoulet@torproject.org>

I believe that this should never trigger, but if it does, it
suggests that there was a gap between is_sensitive_dir_purpose and
purpose_needs_anonymity that we need to fill.  Related to 20077.

Conflict in entrynodes.c: any_bridge_supports_microdescriptors was
removed in master, and modified in 19858_v2

Patch from pingl; patch for 13827.

"ours" commit to avoid bumping version.

("ours" merge to avoid taking version bump)

This helps protect against bugs where any part of a buf_t's memory
is passed to a function that expects a NUL-terminated input.

It also closes TROVE-2016-10-001 (aka bug 20384).

This commit adds or improves the module-level documenation for:

  buffers.c circuitstats.c command.c connection_edge.c control.c
  cpuworker.c crypto_curve25519.c crypto_curve25519.h
  crypto_ed25519.c crypto_format.c dircollate.c dirserv.c dns.c
  dns_structs.h fp_pair.c geoip.c hibernate.c keypin.c ntmain.c
  onion.c onion_fast.c onion_ntor.c onion_tap.c periodic.c
  protover.c protover.h reasons.c rephist.c replaycache.c
  routerlist.c routerparse.c routerset.c statefile.c status.c
  tor_main.c workqueue.c

In particular, I've tried to explain (for each documented module)
what each module does, what's in it, what the big idea is, why it
belongs in Tor, and who calls it.  In a few cases, I've added TODO
notes about refactoring opportunities.

I've also renamed an argument, and fixed a few DOCDOC comments.