A couple of tweaks to the previous re-factor:

- Some of what was defined as "generic itimer" behavior turned out to be
  ptimer-specific.  As such, everything related to the "fired timer queue"
  is now specific to ptimers, and the queue and softint handle fields of
  itimer_ops are not needed.

- Split itimer_fini() into 2 parts: itimer_poision() marks the timer as
  dead and attempts to cancel it.  itimer_fini() is then just responsible
  for freeing itimer resources and releasing the lock.  They are split
  into two parts, as ptimers require an addition processing step between
  those two operations, but other kinds of itimers do not necessarily require
  that.

- Export a few more itimer-related symbols that other itimer types will
  need.

Riding previous kernel version bump since there are no external uses of
this code since the version bump that accompanied the original change.

(thorpej)