vga: add some optional CGA compatibility hacks

This patch adds some optional compatibility hacks (default
disabled) to allow Microport UNIX to function under qemu.

I've tried to structure it to be easy to add more hacks for other
old CGA programs, if anyone ever needs them.

Microport UNIX System V/386 v 2.1 (ca 1987) tries to program
the CGA registers directly with neither the assistance of BIOS, nor
with proper handling of EGA/VGA-only registers.  Note that it didn't
work on real VGA hardware, either (although in that case, the most
obvious problems seemed to be out-of-range hsync and/or vsync
signalling, rather than the issues in this patch).

Eventually real MDA and/or CGA support might provide an alternative to
this patch, although a hybrid approach like this patch might still
be useful in marginal cases.

Signed-off-by: Matthew Ogilvie <mmogilvi_qemu@miniinfo.net>
Signed-off-by: malc <av1474@comtv.ru>
