Make sure we initialize dbus thread primitives

Make sure we initialize dbus thread primitives as apps may
(and do) call g-k in threads. This isn't 100% safe, because
someone might have been doing dbus calls before without
initializing threads. However, there is no safe solution to
this other than just making all such places initialze threads
(or port to GDBus), so we do it anyway. (GVfs does it too, so
most apps end up doing it that way.)

Tweaked by Stef Walter.

https://bugzilla.gnome.org/show_bug.cgi?id=659162