Screen/Linux: Detect powered down NVidia GPU's and refrain from using them.

On hybrid-graphics Optimus Laptops with combined Intel + NVidia GPU,
the NVidia card is usually powered down, therefore offline/disconnected
from the bus. As we no longer low-level attach to Intel-GPU's by default
for safety reasons, Screen() would fall back to the disabled NVidia card,
read trash data from its offlined registers and clutter the console with
many useless warnings.

Now we detect if we try to access an offline NVidia card (register reads
return 0xffffffff) and report/handle this properly by detaching from that
card.

-> No low-level access to either the Intel card or NVidia on such a
   setup. Good, because we don't need low-level access as the Intel
   card gets its timestamps from DRI/OML, and this way we avoid
   confusing users with tons of false alarm warnings.

-> Screen for 64-Bit Octave on Linux rebuilt and lightly tested:
   - Confirmed that it still works correctly on non-Optimus.
   - Don't have a Optimus test machine, so i hope it will work as
     expected on such a Optimus setup. Maybe Stephan de la Rosa can
     confirm.