- Fix spinlock exits to properly handle the recursion count and only

- Fix spinlock exits to properly handle the recursion count and only
  re-enable interrupts when actually releasing the lock.
- Bring across some fixes to propagate_priority from the x86 code.
  (It still doesn't work properly, however.)
- Use the SMTX state when putting a process that blocks on a mutex to sleep.
- Use mi_switch instead of cpu_switch so that accounting works properly as
  well as other things.
- Bring across DDB protection of the spinlock timeout panic which is useful
  in a multiple CPU system when 1 CPU enters the debugger holding the
  sched_lock so that the other CPU doesn't panic as well resulting in all
  sorts of fun things.
- Bring across various other small changes in format strings and comments
  to sync up with the x86 code.