jabber: Reply unknown IQs with service-unavailable instead
It was sending 'feature-not-implemented' for <query> with unknown xmlns
(which makes sense, but the RFC says that's wrong. idk.) and nothing at
all for IQs that don't have query/ping/time elements or an xmlns
attribute. Both get service-unavailable now.
Addresses the rest of trac ticket 533:
https://bugs.bitlbee.org/bitlbee/ticket/533
