kernel - Add a workaround for tty_token during low-level boot
* The low level cninit() code dives into dev/video and does a ton
of tty_token acquisitions and releases, and even after the bug
fixes there is still something weird going on in there.
This is a workaround to wrap a master tty_token around the
cninit() code which prevents an early-boot crash if
lwkt_gettoken()'s td_mpcount optimization is turned off.
(the optimization masks the problem).
* Also assert that the mpcount remains correct after the mess
is done initializing, if it is wrong the SMP/AP boot will
blow up on us.