Commonalize process startup code.

Move common code, that was duplicated in every postmaster child/every
standalone process, into two functions in miscinit.c.  Not only does
that already result in a fair amount of net code reduction but it also
makes it much easier to remove more duplication in the future. The
prime motivation wasn't code deduplication though, but easier addition
of new common code.

src/backend/postmaster/pgarch.c

@@ -157,12 +157,11 @@ pgarch_start(void)
 #ifndef EXEC_BACKEND
 		case 0:
 			/* in postmaster child ... */
+			InitPostmasterChild();
+
 			/* Close the postmaster's sockets */
 			ClosePostmasterPorts(false);
 
-			/* Lose the postmaster's on-exit routines */
-			on_exit_reset();
-
 			/* Drop our connection to postmaster's shared memory, as well */
 			dsm_detach_all();
 			PGSharedMemoryDetach();
@@ -221,21 +220,6 @@ pgarch_forkexec(void)
 NON_EXEC_STATIC void
 PgArchiverMain(int argc, char *argv[])
 {
-	IsUnderPostmaster = true;	/* we are a postmaster subprocess now */
-
-	MyProcPid = getpid();		/* reset MyProcPid */
-
-	MyStartTime = time(NULL);	/* record Start Time for logging */
-
-	/*
-	 * If possible, make this process a group leader, so that the postmaster
-	 * can signal any child processes too.
-	 */
-#ifdef HAVE_SETSID
-	if (setsid() < 0)
-		elog(FATAL, "setsid() failed: %m");
-#endif
-
 	InitializeLatchSupport();	/* needed for latch waits */
 
 	InitLatch(&mainloop_latch); /* initialize latch used in main loop */