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

fix miscounting in LUSERS

assorted fixes

The server should never crash during rehash, even if the config is invalid.

Previously, we generated and prepended a long salt before generating
password hashes. This resulted in the hash verification cutting off long
before it should do. This form of salting is also not necessary with
bcrypt as it's provided by the password hashing and verification
functions themselves, so totally rip it out.

This commit also adds the functionality for the server to automagically
upgrade users to use the new hashing system, which means better
security and more assurance that people can't bruteforce passwords.

No need to apply a database upgrade to do this, whoo! \o/

refactor rehash to rely more on server.config

simplify CAS logic a bit

some tweaks to account/channel unregistration

implement #263