When allocating a new keyboard at vt_upgrade() time, unwind any cngrabs

done on the old keyboard and then do the corresponding number of grabs
on the new keyboard.

This fixes a race that can leave the system with a non-functioning
keyboard.  It goes like this...

 - The bios claims there is an AT keyboard, atkbd attaches.
 - SI_SUB_INT_CONFIG_HOOKS runs.
 - USB probes devices. Devices begin attaching, including disks.
 - GELI prompts for a password for a just-attached disk, which results
   in a cngrab() while atkbd is the keyboard.
 - A USB keyboard attaches.
 - vt_upgrade() runs and switches the keyboard to the new USB keyboard,
   but because cngrab was never called for it, it's not activated and
   keystrokes are ignored.
 - Now there is no functional keyboard and no way to get one; even
   plugging in a different USB keyboard doesn't help, because the console
   is still grabbed, still waiting for a GELI pw.

Discussed with:	     ray@

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