kernel - Improve regressions in usched_dfly (2)

kernel - Improve regressions in usched_dfly (2)

* Allow various fork() behaviors to be supported via
  kern.usched_dfly.features.

* Set the default to place the newly forked process on
  a random cpu instead of the current cpu.

  The bsd4 scheduler had a global queue and could just signal
  a random helper to pick up the thread.  The dfly scheduler
  has per-cpu queues and must actually enqueue the thread to
  another cpu.

  The bsd4 scheduler is still slightly superior here because
  if the parent running on the current cpu immediately waits
  for the child, the child is able to run on the current cpu.
  However, randomization works quite well and this removes
  nearly all of the make -j N regression.