kernel - Improve regressions in usched_dfly (1)

kernel - Improve regressions in usched_dfly (1)

* The new scheduler is MP locked at a very fine-grain.  The old scheduler
  had a global spinlock which effectively serialized competing cores during
  exit/wait sequences.

  With the new scheduler this serialization is gone and resulted in a
  vfork performance regression due to a fallback tsleep loop in the
  reaper.

* This fixes the problem with an explicit signal bit for tsleep/wakeup.
  The sequence is avoided if the reaper determines the thread has
  already completed its exit.