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.

Closes bug #155.
Merge branch 'bug155-allowAtInUser'

* bug155-allowAtInUser:
  Allow "@" character in user names for authentication