Cause ShutdownPostgres to do a normal transaction abort during backend

exit, instead of trying to take shortcuts.  Introduce some additional
shutdown callback routines to eliminate kluges like having ProcKill
be responsible for shutting down the buffer manager.  Ensure that the
order of operations during shutdown is predictable and what you would
expect given the module layering.

src/backend/postmaster/pgstat.c

@@ -13,7 +13,7 @@
  *
  *	Copyright (c) 2001-2005, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.102 2005/07/29 19:30:04 tgl Exp $
+ *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.103 2005/08/08 03:11:40 tgl Exp $
  * ----------
  */
 #include "postgres.h"
@@ -665,7 +665,7 @@ pgstat_report_autovac(void)
  * pgstat_bestart() -
  *
  *	Tell the collector that this new backend is soon ready to process
- *	queries. Called from tcop/postgres.c before entering the mainloop.
+ *	queries. Called from InitPostgres.
  * ----------
  */
 void
@@ -686,7 +686,7 @@ pgstat_bestart(void)
 	 * Set up a process-exit hook to ensure we flush the last batch of
 	 * statistics to the collector.
 	 */
-	on_proc_exit(pgstat_beshutdown_hook, 0);
+	on_shmem_exit(pgstat_beshutdown_hook, 0);
 }
 
 /* ---------
@@ -738,9 +738,7 @@ pgstat_report_analyze(Oid tableoid, bool shared, PgStat_Counter livetuples,
 /*
  * Flush any remaining statistics counts out to the collector at process
  * exit.   Without this, operations triggered during backend exit (such as
- * temp table deletions) won't be counted.  This is an on_proc_exit hook,
- * not on_shmem_exit, so that everything interesting must have happened
- * already.
+ * temp table deletions) won't be counted.
  */
 static void
 pgstat_beshutdown_hook(int code, Datum arg)