jabber: Validate user moods, and make the /mood cmd behave flexibly.

A user in Pidgin running "/mood ?" or "/mood -" would result in invalid
XML being sent to the server (similar to #14342, except '<-/>' or '').

Prevent this by ensuring the user is specifying something from the list.
The /mood command will also now try to treat its entire arguments as
a single string, and set that as the mood -- I figure this is what a
user would expect?