Improve irc-msg support for multiple connections

Right now, if I'm connected to two networks, am connected to #foo on only one of them, and I 'irc-msg #foo', BarnOwl will still ask me to specify a network identifier. That's silly. Reworked the code so that it picks up the network automatically if it is unique.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>