cmd: add support for showing placeholder commands/opts/args in command usage strings

cmd/command.h

@@ -51,10 +51,16 @@ struct b_command_usage_arg {
 	struct b_command_arg *arg;
 };
 
+struct b_command_usage_command {
+	struct b_queue_entry cmd_entry;
+	unsigned int cmd_id;
+};
+
 struct b_command_usage {
 	b_command_usage_flags u_flags;
 	struct b_queue_entry u_entry;
 
+	struct b_queue u_cmd;
 	struct b_queue u_opt;
 	struct b_queue u_arg;
 };