Implement RFC 2812 handling of "0" argument to JOIN

The students in my software-engineering class are writing IRC clients in
Java, and I'm running ngIRCd as a sandbox for them to play in. We
noticed ngIRCd doesn't obey the "JOIN 0" command specified in RFC 2812:

   JOIN 0             ; Leave all currently joined
                      channels.

http://tools.ietf.org/html/rfc2812#section-3.2.1
I believe the following patch addresses this. Cheers!

[fw@strlen.de: put it into a seperate function]