macro wrappers for g_once_init_enter/leave

macro wrappers for g_once_init_enter/leave

Give the macro wrapper treatment to g_once_init_enter() and leave() in
the same style that we did for gatomic.

It is now possible to use these macros with any pointer-sized object,
and not just gsize.  The leave() macro ensures that the initialisation
result is a compatible type with the pointer that it is being written
to.

Just like with gatomic, there could be problems caused by use of (void*)
casts.  We'll see how that goes, and reevaluate if necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=660743