Allow commands without any additional arguments, as stated in the

manpage.  While here, exit early when there is nothing to do.

PR:            168415
Submitted by:  Zhihao Yuan (initial version)
MFC after:     1 week
Approved by:   kib (mentor)

usr.bin/stdbuf/stdbuf.c

@@ -72,8 +72,8 @@ main(int argc, char *argv[])
 	}
 	argc -= optind;
 	argv += optind;
-	if (argc < 2)
-		usage(0);
+	if (argc == 0)
+		exit(0);
 
 	if (ibuf != NULL && setenv("_STDBUF_I", ibuf, 1) == -1)
 		warn("Failed to set environment variable: %s=%s",