Add a new ioctl to vt to make it possible to export RGB offsets
set by vt drivers.
This is intended to be used by xf86-video-scfb driver,
to properly set the RGB masks, as in
This fixes wrong colors in X on PowerPC64 (big-endian) machines
using scfb/ofwfb (e.g. Talos II and Blackbird) and also allows
other drivers to export their RGB offsets.