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.

helpdir = $(datadir)/mcabber/help

install-data-local:
	$(INSTALL) -d $(DESTDIR)$(datadir)/mcabber/help
	for lang in en de fr nl pl ru uk; do \
		$(INSTALL) -d $(DESTDIR)$(datadir)/mcabber/help/$$lang; \
		$(INSTALL) -m 0644 $(srcdir)/$$lang/* \
		           $(DESTDIR)$(datadir)/mcabber/help/$$lang; \
	done