[AD] using internals from addons

[Peter Wang <novalazy@xxxxxxxxxx>]

How do we feel about using Allegro internals in addons?  I think
kcm_audio objects should be automatically destroyed when
al_uninstall_audio() is called.  This implies reusing the code in
src/dtor.c, although we could make a copy.  We do use vector.c in
kcm_audio already.

Do we want al_uninstall_audio to be called when Allegro shuts down?
This implies calling _al_add_exit_func, _al_remove_exit_func which are
internal.

Peter