Add more sources of nick!user@host updates

Update IrcUser user and hostname information from more sources.  This
is more important now since away-notify disables automatic WHO
polling, so user@host information won't be updated if the server
doesn't generate fake join/quit messages for /chghost and friends.

In the future, IRCv3 capability chghost will remove the need for this
once all servers support it.  (In only a few decades, right..?)
For chghost, see http://ircv3.net/specs/extensions/chghost-3.2.html