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.