* be NULL if no reply is needed for the given command).
* @return 0 if successful, or errors {USTCMD_ERR_ARG, USTCMD_ERR_CONN}
*/
* be NULL if no reply is needed for the given command).
* @return 0 if successful, or errors {USTCMD_ERR_ARG, USTCMD_ERR_CONN}
*/