* Call self._schedule() from inside Scheduler._poll(). This ensures that

* Call self._schedule() from inside Scheduler._poll(). This ensures that
  new jobs can scheduled as soon a the load average is low enough, and
  one of the running jobs doesn't have to complete for _schedule() to
  get called.

* Cache the result of _background_mode() so it doesn't have to be called
  inside _schedule(), since this method is called a lot more often now.

* Move background related state adjustments into _background_mode().

* Call _status_display.display() unconditionally inside _schedule_tasks(),
  since we want to update the display to show current load average even
  if no new jobs are added.

svn path=/main/trunk/; revision=11135