powerd - Add options to set poll_rate and sample_time, adjust idle algorithm

powerd - Add options to set poll_rate and sample_time, adjust idle algorithm

* Add -r poll_rate and -T sample_time.  The user can adjust the values
  to e.g. make a workstation react more quickly to UI operations coming
  out of idle and to take longer to return to idle.

* Use a shorter averaging time (3 poll intervals) for ratcheting up the
  speed and a longer averaging time (adjustable by -T) for ratcheting down
  the speed.

* Enhance the manual page, describe default values and make suggestions.