Hide things not appropriate behind is_backend

Also add explicit command to exit the server, FIO_QUIT.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/init.c b/init.c
index 5d57091..ce16450 100644
--- a/init.c
+++ b/init.c
@@ -1317,7 +1317,7 @@
 		exit(exit_val);
 
 	if (is_backend)
-		fio_server();
+		return fio_server();
 
 	if (td) {
 		if (!ret)
@@ -1367,6 +1367,8 @@
 			return 0;
 		if (exec_profile)
 			return 0;
+		if (is_backend)
+			return 0;
 
 		log_err("No jobs(s) defined\n\n");
 		usage(argv[0]);