kernel - Fix rare IPIQ freezes

kernel - Fix rare IPIQ freezes

* Ensure that an IPI interrupt is sent went waiting for an IPIQ
  to drain.  The IPIQ can be pushed up by passive IPIs and not
  necessarily have a signal pending on the target cpu, so we
  have to put the check in our drain loop rather than outside
  the loop.

* Add a cpu_pause() to reduce power use for the IPIQ drain case.

* Normalize the use of gd_intr_nesting_level and clean up the
  code syntax for the ipiq_optimized case.

* Remove the previous temporary IPI interrupt signaling fix, it
  was incomplete.

* Fix a missing crit_exit() in the ENOENT case for
  lwkt_send_ipiq3_nowait().

* Track cpu's which are in the middle of ipiq processing and
  assert that a cpu is not in an IPIQ processing loop when
  switching between threads.

* Normalize the use of ip->ip_npoll in the IPIQ code.  This
  field is used to avoid unnecessary IPI interrupts.