Make it easy to detach completely from shared memory.

The new function dsm_detach_all() can be used either by postmaster
children that don't wish to take any risk of accidentally corrupting
shared memory; or by forked children of regular backends with
the same need.  This patch also updates the postmaster children that
already do PGSharedMemoryDetach() to do dsm_detach_all() as well.

Per discussion with Tom Lane.

src/backend/postmaster/pgstat.c

@@ -50,6 +50,7 @@
 #include "postmaster/postmaster.h"
 #include "storage/proc.h"
 #include "storage/backendid.h"
+#include "storage/dsm.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"
 #include "storage/latch.h"
@@ -709,6 +710,7 @@ pgstat_start(void)
 			on_exit_reset();
 
 			/* Drop our connection to postmaster's shared memory, as well */
+			dsm_detach_all();
 			PGSharedMemoryDetach();
 
 			PgstatCollectorMain(0, NULL);