Allow dynamic allocation of shared memory segments.

Patch by myself and Amit Kapila.  Design help from Noah Misch.  Review
by Andres Freund.

src/backend/storage/ipc/ipci.c

@@ -30,6 +30,7 @@
 #include "replication/walreceiver.h"
 #include "replication/walsender.h"
 #include "storage/bufmgr.h"
+#include "storage/dsm.h"
 #include "storage/ipc.h"
 #include "storage/pg_shmem.h"
 #include "storage/pmsignal.h"
@@ -249,6 +250,10 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
 		ShmemBackendArrayAllocation();
 #endif
 
+	/* Initialize dynamic shared memory facilities. */
+	if (!IsUnderPostmaster)
+		dsm_postmaster_startup();
+
 	/*
 	 * Now give loadable modules a chance to set up their shmem allocations
 	 */