Implement RFC2812 JOIN 0

JOIN 0 does not happen in RFC1459, but in RFC2812. Despite that fact, the implementation in ircds predate RFC1459 since when channels where numbered.

There's still people using this feature in order to part their bouncers from channels when going offline (query only), and irssiproxy.

In order to minimize the risk of abuse, I don't to proper channel list evaluation for join, but only accept the single syntax "JOIN 0".

Also integrated most of the suggestions from the last pull request.