kernel - Refactor Xinvltlb a little, turn off the idle-thread invltlb opt

kernel - Refactor Xinvltlb a little, turn off the idle-thread invltlb opt

* Turn off the idle-thread invltlb optimization.  This feature can be
  turned on with a sysctl (default-off) machdep.optimized_invltlb.  It
  will be turned on by default when we've life-tested that it works
  properly.

* Remove excess critical sections and interrupt disablements.  All entries
  into smp_invlpg() now occur with interrupts already disabled and the
  thread already in a critical section.  This also defers critical-section
  1->0 transition handling away from smp_invlpg() and into its caller.

* Refactor the Xinvltlb APIs a bit.  Have Xinvltlb enter the critical
  section (it didn't before).  Remove the critical section from
  smp_inval_intr().  The critical section is now handled by the assembly,
  and by any other callers.

* Add additional tsc-based loop/counter debugging to try to catch problems.

* Move inner-loop handling of smp_invltlb_mask to act on invltlbs a little
  faster.

* Disable interrupts a little later inside pmap_inval_smp() and
  pmap_inval_smp_cmpset().