kernel - Add usched_dfly algorith, set as default for now (4)

kernel - Add usched_dfly algorith, set as default for now (4)

* Fix fork regression with usched_dfly.  Most fork/exec sequences involve
  the parent waiting.  The new scheduler was placing the newly forked
  process on another cpu which is non-optimal if the parent is going
  to immediately wait.

  Instead if there is nothing else waiting to run on the current cpu,
  leave the forked process on the current cpu initially.  If the parent
  waits quickly the forked process will get cpu, otherwise it will get
  scheduled away soon enough.  If the parent forks additional children
  then we find there is something on the queue now (the first child) and
  put the additional children on other cpus.

Reported-by: thesjg