Make libvgl mostly work without superuser privilege in direct modes by

Make libvgl mostly work without superuser privilege in direct modes by
not doing any unnecessary PIO instructions or refusing to start when the
i/o privilege needed for these instructions cannot be acquired.

This turns off useless palette management in direct modes.  Palette
management had no useful effect since the hardware palette is not used
in these modes.

This transiently acquires i/o privilege if possible as needed to give
VGLSetBorder() and VGLBlankDisplay() a chance of working.  Neither has
much chance of working.  I was going to drop support for them in direct
modes, but found that VGLBlankDisplay() still works with an old graphics
card on a not so old LCD monitor.

This has some good side effects: reduce glitches for managing the palette
for screen switches, and speed up and reduce async-signal-unsafeness in
mouse cursor drawing.