Allow "@" character in user names for authentication
The "@" character isn't allowed in IRC usernames, because it is the
separator between user name and hostname in IRC masks:
<nickname>!<username>@<hostname>
This patch accepts user names including "@" characters, saves the
unmodified name for authentication but stores only the part in front
of the "@" character as "IRC user name". And the latter is how
ircd2.11, Bahamut, and irc-seven behave as well.