[9fans] vga bios mode support, on-the-fly screen resizing

[Russ Cox <russcox@gmail.com>]

Plan 9 now supports using the VESA BIOS to set video modes.

There is a new aux/vga binary that you need to pull from sources.
Then grab a kernel from /n/sources/contrib/rsc/vesa/9/pc.
I've built 9pc 9pcf 9pcdisk and 9pccpuf.

Please test them out and then send me mail telling me:
 - type of video card you have
 - pci ids of video card: pci | grep ' 03\.'
 - what worked and what did not

Things to try include:
 - aux/vga -m vesa -p (should print available modes)
 - aux/vga -m vesa -l 1024x768x16 (switch to 1024x768x16)
 - aux/vga -l text (switch back to text mode)

Independent of the VESA support, the kernel can now change the screen
size/depth at any time, even while rio is running.  Just run aux/vga
-l new-size.

If you run aux/vga -l text while rio is running, you will switch to text mode
but not have a prompt (rio is still running, and the shell is waiting for
it to exit).  As long as you haven't clicked the mouse to change to another
window you should be able to blindly type aux/vga -l some-vga-size to
get back to rio.

Thanks to Roger Peppe, Fred Bonfanti, and Devon O'Dell for help
getting this far.

Russ