add validation for isupport tokens

According to jwheare, ISO 8601 doesn't specify the behavior one way
or the other, and the server-time spec is also ambiguous, but this
is safest.

history replay enhancements

fix hostserv help string

fix #313 and a related issue

draft/resume-0.2 implementation, message history support

Replace docopt.Parse with docopt.ParseArgs

docopt.Parse has been deprecated in
943c2addfe

implement NS PASSWD for password changes

fix #306

Fix spurious bidi rule violations in casefolding channel names
by stripping the # before starting the casefolding.

* fix a race condition: a call to `Write` does not spawn a writer goroutine
  if the trylock is held, so `BlockingWrite` must check for fresh data after
  releasing the trylock
* streamline some close/finalize logic

prevents default create/open of buntdb, requires user to call initdb

fix PROXY protocol support for IPv6

Correct NS REGISTER example

1. Handle PROXY lines with IPv6 addresses starting with ::
(similar to WEBIRC in issue #211)

2. Strip v6 mapping from v4 addresses when handling proxied IPs.

configurable file mode for unix socket listeners

re-add nil receiver for ModeSet.HasMode

new password hashing technique

Fix a bug in 330 RPL_WHOISACCOUNT

fix a crash when SAMODE'ing in a channel you're not joined to

According to https://defs.ircdocs.horse/defs/numerics.html, 330 RPL_WHOISACCOUNT
takes 4 parameters: `<client> <nick> <authname> :<info>`. We were omitting
the second parameter (the target nick).

Also refactor locking.

Miscellaneous cleanup