kernel - Break up scheduler and loadavg callout

kernel - Break up scheduler and loadavg callout

* Change the scheduler and loadavg callouts from cpu 0 to all cpus, and
  adjust the allproc_scan() and alllwp_scan() to segment the hash table
  when asked.

  Every cpu is now tasked with handling the nominal scheduler recalc and
  nominal load calculation for a portion of the process list.  The portion
  is unrelated to which cpu(s) the processes are actually scheduled on,
  it is strictly a way to spread the work around, split up by hash range.

* Significantly reduces cpu 0 stalls when a large number of user processes
  or threads are present (that is, in the tens of thousands or more).  In
  the test below, before this change, cpu 0 was straining under 40%+
  interupt load (from the callout).  After this change the load is spread
  across all cpus, approximately 1.5% per cpu.

* Tested with 400,000 running user processes on a 32-thread dual-socket
  xeon (yes, these numbers are real):

  12:27PM  up 8 mins,  3 users, load avg: 395143.28, 270541.13, 132638.33
  12:33PM  up 14 mins, 3 users, load avg: 399496.57, 361405.54, 225669.14

* NOTE: There are still a number of other non-segmented allproc scans in
	the system, particularly related to paging and swapping.

* NOTE: Further spreading-out of the work may be needed, by using a more
  	frequent callout and smaller hash index range for each.