On Darwin, refuse postmaster startup when multithreaded.

The previous commit introduced its report at LOG level to avoid
surprises at minor release upgrade time.  Compel users deploying the
next major release to also deploy the reported workaround.

src/backend/postmaster/postmaster.c

@@ -1216,7 +1216,7 @@ PostmasterMain(int argc, char *argv[])
 	 * normal case on Windows, which offers neither fork() nor sigprocmask().
 	 */
 	if (pthread_is_threaded_np() != 0)
-		ereport(LOG,
+		ereport(FATAL,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("postmaster became multithreaded during startup"),
 		 errhint("Set the LC_ALL environment variable to a valid locale.")));
@@ -4781,11 +4781,14 @@ ExitPostmaster(int status)
 	/*
 	 * There is no known cause for a postmaster to become multithreaded after
 	 * startup.  Recheck to account for the possibility of unknown causes.
+	 * This message uses LOG level, because an unclean shutdown at this point
+	 * would usually not look much different from a clean shutdown.
 	 */
 	if (pthread_is_threaded_np() != 0)
 		ereport(LOG,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("postmaster became multithreaded")));
+				(errcode(ERRCODE_INTERNAL_ERROR),
+				 errmsg_internal("postmaster became multithreaded"),
+		   errdetail("Please report this to <pgsql-bugs@postgresql.org>.")));
 #endif
 
 	/* should cleanup shared memory and kill all backends */