The first stage of console driver reorganization: activate new

The first stage of console driver reorganization: activate new
keyboard and video card drivers.

Because of the changes, you are required to update your kernel
configuration file now!

The files in sys/dev/syscons are still i386-specific (but less so than
before), and won't compile for alpha and PC98 yet.

syscons still directly accesses the video card registers here and
there; this will be rectified in the later stages.