Improve vkernel support.

Improve vkernel support.

* Add a new cothread API for vkernel drivers.  This creates real threads
  to act as DMA engines for I/O.  These threads operate OUTSIDE the
  vkernel's cpu abstraction and thus work with UP builds or SMP builds with
  -n 1.

* Asynchronize the virtual disk (vkd).   VKD now queues I/O to a cothread
  which runs it and then generates a signal to the vkernel's cpu 0 to run
  the virtual device's 'interrupt' function.

* The new cothread code works better then the kqueue code because it allows
  I/O's to be ganged together for bulk operation without causing a signal
  for each one.  The kqueue code is still used by VKE but should
  be considered obsolete.