kernel - Implement QUICKHALT shortcut for unmounting during shutdown

kernel - Implement QUICKHALT shortcut for unmounting during shutdown

* Add the MNTK_QUICKHALT flag which allows the system to just unlink but
  otherwise ignore certain mount types during a halt or reboot.  For now
  we flag tmpfs, devfs, and procfs.

* The main impetus for this is to reduce the messing around we
  do with devfs during a shutdown.  Devfs has its fingers, and its
  vnodes, prettymuch sunk throughout the system (e.g. /dev/null, system
  console, vty's, root mount, and so on and so forth).  There's no
  real need to attempt to unwind all of that mess nicely.