Allow joins from clients

[martinetd]

Currently clients sending JOIN command will just be ignored -- we should check if such a chan exists and initiate a proper join, or reply with an error if it doesn't exist.

(This doesn't seem to cause any real trouble right now so delaying a bit; please report if you think it's an issue -- this could be used as a mechanism to "promote" a 1-to-1 query to chan?)