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.
Only some very ancient distributions don't ship with Libevent 2 anymore,
even the oldest supported Ubuntu LTS version has it. This allows us to
get rid of a lot of compat code.
