kernel - Refactor cpu localization for VM page allocations (3)

kernel - Refactor cpu localization for VM page allocations (3)

* Instead of iterating the cpus in the mask starting at cpu #0, iterate
  starting at mycpu to the end, then from 0 to mycpu - 1.

  This fixes random masked wakeups from favoring lower-numbered cpus.

* The user process scheduler (usched_dfly) was favoring lower-numbered
  cpus due to a bug in the simple selection algorithm, causing forked
  processes to initially weight improperly.  A high fork or fork/exec
  rate skewed the way the cpus were loaded.

  Fix this by correctly scanning cpus from the (scancpu) rover.

* For now, use a random 'previous' affinity for initially scheduling a
  fork.