kernel - Fix degenerate condition in dfly_setrunqueue_locked()

kernel - Fix degenerate condition in dfly_setrunqueue_locked()

* dfly_setrunqueue_locked() was unconditionally adding the lp to the end
  of the round-robin queue for the specified priority due to a degenerate
  '... || !TAILQ_EMPTY()' test.

  Remove the degenerate test, allowing a thread to be entered onto the
  head of the queue if its rrcount is reasonable.  Theoretically this should
  allow interactivity at the same queue priority even in a heavily-batched
  environment.

* Do not reset rrcount when adding to the head of the queue even if
  the queue is otherwise empty.  This allows the round-robin state to be
  retained across short delays (for example, nanosleeps) which are not as
  interactive as the system might otherwise believe, resulting in a more fair
  apportionment of cpu between batchy threads that use different delays but
  wind up at the same priority.

* Also adjust dfly_resetpriority() to reset lwp_rrcount to 0 when moving
  a thread to a higher priority queue.

* These changes should probably be re-tested against the postgres test
  suite to determine if any regressions were introduced.  It could very
  well be that the original action of always adding to the end of the queue
  might actually be the best action to take.

Reported-by: Adrian Drzewiecki <z@drze.net>