Schedule ShutdownXLOG() in single user mode using before_shmem_exit().

Previously on_shmem_exit() was used. The upcoming shared memory stats patch
uses DSM segments to store stats, which can not be used after the
dsm_backend_shutdown() call in shmem_exit().  There does not seem to be any
reason to do ShutdownXLOG() via on_shmem_exit(), so change it.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210405092914.mmxqe7j56lsjfsej@alap3.anarazel.de
Discussion: https://postgr.es/m/20210803023612.iziacxk5syn2r4ut@alap3.anarazel.de

src/backend/utils/init/postinit.c

@@ -638,7 +638,11 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 		/* Reset CurrentResourceOwner to nothing for the moment */
 		CurrentResourceOwner = NULL;
 
-		on_shmem_exit(ShutdownXLOG, 0);
+		/*
+		 * Use before_shmem_exit() so that ShutdownXLOG() can rely on DSM
+		 * segments etc to work (which in turn is required for pgstats).
+		 */
+		before_shmem_exit(ShutdownXLOG, 0);
 	}
 
 	/*