Speed up the boot sequence by deferring the scan of xt keyboard code set.

Speed up the boot sequence by deferring the scan of xt keyboard code set.

If you were wondering why there's a pause of 5 seconds after loading the
kernel and before seeing the copyright in blue when booting some of your
machines, well this is for you!

mlarkin@ found that this delay is caused by some code probing for xt scan
code tables when pckbd(4) initialize the console keyboard.  So this diff
implements an idea from deraadt@ to only do the scan once, either during
*attach() or if we enter ddb> or ukc> before that!

ok miod@, mlarkin@, deraadt@, shadchin@