Install an atexit(2) callback that ensures that proc_exit's cleanup processing

Install an atexit(2) callback that ensures that proc_exit's cleanup processing
will still be performed if something in a backend process calls exit()
directly, instead of going through proc_exit() as we prefer.  This is a second
response to the issue that we might load third-party code that doesn't know it
should not call exit().  Such a call will now cause a reasonably graceful
backend shutdown, if possible.  (Of course, if the reason for the exit() call
is out-of-memory or some such, we might not be able to recover, but at least
we will try.)