Improve Private Storage detection. MCabber now correctly detects Private Storage support when receiving an item-not-found error (which doesn't mean the server has no support for P.S., but that it's empty). I've added a return value to the callback functions. For now the return value is only used for IQ error handling.

 /RENAME name

Setzt den Namen des gerade ausgewählten Buddys bzw. der ausgewählten Gruppe auf "name".