Add an atexit handler to remove the D-Bus socket

We should try not to leave stale sockets and their directories when exiting.
They are removed when calling atk_bridge_adaptor_cleanup, but gtk does not
currently call this function when exiting, and there is not a good place
to call it from gtk.

https://bugzilla.gnome.org/show_bug.cgi?id=684076
Release v2.12.1