pgsql: Install an atexit(2) callback that ensures that proc_exit's

Log Message:
-----------
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.)

Modified Files:
--------------
pgsql/src/backend/storage/ipc:
ipc.c (r1.102 -> r1.103)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/ipc.c?r1=1.102&r2=1.103)