cmd: add functions to report option/arg errors to the public API

cmd/option.c

@@ -231,3 +231,22 @@ void z__b_get_option_usage_string(
 		}
 	}
 }
+
+struct b_command_arg *b_command_option_get_arg_with_id(
+	struct b_command_option *opt, unsigned int id)
+{
+	b_queue_iterator it;
+	b_queue_foreach (&it, &opt->opt_args) {
+		struct b_command_arg *arg
+			= b_unbox(struct b_command_arg, it.entry, arg_entry);
+		if (!arg) {
+			continue;
+		}
+
+		if (arg->arg_id == id) {
+			return arg;
+		}
+	}
+
+	return NULL;
+}