Updated VGA driver; from NetBSD. Needed for wscons on i386 and alpha. These

files could probably be updated even a bit further (they are from mid-summer).

In addition, I've added support for console scrollback, somewhat inspired by
Linux's vgacon driver. Basically, instead of allocating our own buffer and
doing lots of copies, we take advantage of Video RAM and just modify the VGA
display origin register as appropriate. This approach has a few advantages:
simple to implement, no wasted KVM, it's fast, and after a boot you can now
scroll back all the way to the BIOS messages (assuming your msgbuf is of a
typical length :). Disadvantages are that the VRAM buffer is relatively
small (only 32k) and we do not support raster devices through this method.
(thanks to mickey@ for pointing this out).

The code for this is fairly unobtrusive, so should we come up with a better
approach to console scrollback at a later time (i.e., even more platform
independent) it should be easy to revert this.

We're one step further in porting nice features of PCVT over to wscons.

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