taskqueue - Major overhaul

taskqueue - Major overhaul

* Move taskqueue from critical sections to fine-grained locking with
  lockmgr for the list of taskqueues (could be changed to spinlocks) and
  spinlock for the task queue in each taskqueue itself.

* Add a taskqueue_swi_mp which is for mpsafe tasks. Unlike taskqueue_swi
  the mplock is not acquired when tasks are run.

* Add FreeBSD's taskqueue_start_threads and family, allowing for
  per-taskqueue threads.

* Add FreeBSD's taskqueue_block and taskqueue unblock.

* Out of necessity add a register_swi_mp, which registers a swi marked
  as MPSAFE.

Partially-Obtained-from: FreeBSD