Fix a number of SMP issues.

Fix a number of SMP issues.

* Add required pause instructions in code paths that skip running "HLT".  This
  occurs when e.g. cpu #1 is running code with the BGL while cpu #2's only
  runnable thread requires the BGL.  cpu #2's LWKT scheduler spins in that
  case.  Similar situations can occur when the only runnable threads on a
  cpu are waiting for a token.

* Add required pause instructions to spin loops.  DragonFly has very few spin
  locks (e.g. things like com_lock()) but its a good idea anyway to avoid
  known livelock issues on Intel cpus.

* Fix a pending interrupt / HLT race.  We were not atomically retiring
  pending interrupts prior to potentially HLTing the cpu.  This could
  result in an SMP machine's network locking up until a key is hit on
  the console, then magically resuming.

Lockups-reported-by: Peter Avalos <pavalos@theshell.com>