vga(4): Simplify conditions for mode rejection somewhat.

vga(4): Simplify conditions for mode rejection somewhat.

When mapping the video parameter table to our mode_map, only reject modes
which are neither 40x25, 80x25 nor 80x30, instead of rejecting or accepting
the whole table based on comparing the initial mode's BIOS and power-up
register settings.

Add a VGA_KEEP_POWERON_MODE option which (when set) preserves the register
settings which were found for the initial mode upon power-on for switching
back to it (or some mode derived from it) from another mode. The power-on
settings are also taken for the initial mode in case the video parameter
table has no valid settings for it.

This is kinda similar to what's proposed in FreeBSD's kern/64114.