kernel - Add usched_dfly algorith, set as default for now (8)

kernel - Add usched_dfly algorith, set as default for now (8)

* Fix additional edge cases, in particular improving the process pairing
  algorithm to reduce flapping.

* Reorder conditionals in dd->uschedcp assignment to improve the hot path.

* Rewrite the balancing rover.  The rover will now move one process per
  tick from a very heavily loaded cpu queue to a lightly loaded cpu queue.
  Each cpu target is iterated by the rover, one target per tick.

* Reformulate dfly_chooseproc_locked() and friends.  Add a capability to
  choose the 'worst' process (from the end of the queue), which is used
  by the rover.

* When pulling a random thread we require the queue it is taken from to
  be MUCH more heavily loaded than our own queue, which avoids ping-ponging
  processes back and forth when the load is not balanced against the number
  of cpu cores (e.g. 6 servers, 4 cores).