Sync IrcUser account, add WHOX for account-notify

Add 'account' field to IrcUser, tracking logged-in account, e.g.
NickServ/SASL.  In testing, this works fine with older/newer
cores/servers.

Implement support for WHOX as in IRCv3 specifications, determining
logged-in accounts on first join to channels.  Mimics HexChat's
ordering of fields, reusing any testing they've done.

Display WHOX replies as [WhoX].  As fields are user-specified, more
processing would be difficult to manage.  WHO replies are unchanged.

Unify WHO reply processing in a common function.

Preliminary test results on Ubuntu:
> New core, new client - works fine
> New core, old client - client prints warnings to console but works
(Complains of no matching slot for sync call.  Old client doesn't
use the new information, though.)
> New client, old core - works fine, assumes unknown account state
> New core, Quasseldroid alpha - works fine, warnings in debug log

See http://faerion.sourceforge.net/doc/irc/whox.var
And https://github.com/hexchat/hexchat/blob/c874a9525c9b66f1d5ddcf6c4107d046eba7e2c5/src/common/proto-irc.c#L750