sys/: Set shared memory segments to be deleted as soon as we have attached, that way they get cleaned up automaticall...

Original commit message from CVS:
* sys/ximage/ximagesink.c: (gst_ximagesink_check_xshm_calls),
(gst_ximagesink_ximage_new), (gst_ximagesink_ximage_destroy):
* sys/xvimage/xvimagesink.c: (gst_xvimage_buffer_destroy),
(gst_xvimagesink_check_xshm_calls), (gst_xvimagesink_xvimage_new):
Set shared memory segments to be deleted as soon as we have attached,
that way they get cleaned up automatically if we crash.