yanglint FEATURE hints for option arguments

... and also completion if prefix is also specified.
diff --git a/tools/lint/cmd_load.c b/tools/lint/cmd_load.c
index f5883e9..af204ca 100644
--- a/tools/lint/cmd_load.c
+++ b/tools/lint/cmd_load.c
@@ -62,7 +62,7 @@
         goto cleanup;
     }
 
-    while ((opt = getopt_long(argc, argv, "F:hi", options, &opt_index)) != -1) {
+    while ((opt = getopt_long(argc, argv, commands[CMD_LOAD].optstring, options, &opt_index)) != -1) {
         switch (opt) {
         case 'F': /* --features */
             if (parse_features(optarg, &fset)) {