Set MaxBackends only on bootstrap and standalone modes

... not on auxiliary processes.  I managed to overlook the fact that I
had disabled assertions on my HEAD checkout long ago.

Hopefully this will turn the buildfarm green again, and put an end to
today's silliness.

src/backend/tcop/postgres.c

@@ -3653,6 +3653,11 @@ PostgresMain(int argc, char *argv[], const char *username)
 		 * Create lockfile for data directory.
 		 */
 		CreateDataDirLockFile(false);
+
+		/* In EXEC_BACKEND, this was set via BackendParameters */
+#ifndef EXEC_BACKEND
+		InitializeMaxBackends();
+#endif
 	}
 
 	/* Early initialization */

src/backend/utils/init/postinit.c

@@ -453,8 +453,6 @@ InitializeMaxBackends(void)
 void
 BaseInit(void)
 {
-	InitializeMaxBackends();
-
 	/*
 	 * Attach to shared memory and semaphores, and initialize our
 	 * input/output/debugging file descriptors.