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.

[Desktop Entry]
Encoding=UTF-8
Version=0.9.2-dev
Name=MCabber
Type=Application
Comment=Console Jabber Client
Terminal=true
Exec=mcabber
Path=
Icon=
Categories=ConsoleOnly;System;Application;Network;InstantMessaging
GenericName=Jabber IM Client