the system is shutting down or rebooting.
- Set this global in a new function called kern_reboot(), which is currently
just a basic wrapper around cpu_reboot().
- Call kern_reboot() instead of cpu_reboot() almost everywhere; a few
places remain where it's still called directly, but those are in early
pre-main() machdep locations.
Eventually, all of the various cpu_reboot() functions should be re-factored
and common functionality moved to kern_reboot(), but that's for another day.