vt: emulate 8- and 24-bit colour codes.

Most other mainstream terminals support "xterm256" colours, which means
people sometimes use these blindly without checking capabilities.

Because of hardware limitations of VGA consoles, colours are downgraded to
16 foregrounds and 8 backgrounds.  On fbdev consoles it would be possible
to support them without quality loss, but adding that would require quite a
large amount of code.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>