Regular callback improvements

[JereLeppanen]

• Regular callback is called only if there's no input from the client during the regular interval. Fix by calling the callback when the interval expires, regardless of input from the client.

• A minimum regular callback interval of one second may be too long for some use cases. Add a new API function cli_regular_interval_usec(), which is the same as cli_regular_interval(), except that the time unit is microseconds instead of seconds.