Allow tuning of reader thread ppoll timeout value

Allow tuning of reader thread ppoll timeout value

Default value of 200ms causes too many wake-ups for embedded platforms
low power use-cases. Timeout of several seconds is expected on these.
New option is -T x where x is timeout in ms.

* runtime/staprun/common.c (parse_args): Parse -T option.
  (usage): Mention it.
* runtime/staprun/relay.c (reader_thread): Use -T value to override default
  polling interval.
* runtime/staprun/staprun.8: Document it.