[AD] SVGAlib update

[Peter Wang <tjaden@xxxxxxxxxx>]

I think I got most of the problems I had with this driver ironed
out.  The "Known problems" list is pretty short now:

- After quitting some programs (e.g. demo) you may need to run "stty
  sane" after quitting, like when a curses program crashes.
  Sometimes there's just blinking crap at the bottom..

- vt switching in BACKAMNESIA mode causes font weirdity in text mode
  on a Voodoo Banshee.  The problem goes away when the program quits.

And there's one bit in "Untested":

- If you have a card that thinks in BGR, please try it and tell me
  your results, with both linear and banked (#ifdef if necessary).

Some of the old instabilities were due to the SVGAlib VESA driver,
which uses vm86(), which I later discovered doesn't like to be
interrupted.  So we disable asynchronous events in the right places.

I first used __al_linux_async_exit and async_init.  This works,
except in the VT switching bits, which plays around with the sigalrm
stuff.  So I exposed _sigalrm_stop_timer and start_timer for when
you absolutely can't be interrupted, but can't use _sigalrm_exit and
_sigalrm_init (called by __al_linux_async_exit/init) because they're
too high-level.

The driver itself has changed a lot I didn't send it as a diff. 
If anyone is listening, please test this driver as much as possible,
including the console switching.  It's pretty much _there_ on my
machines, so I'd like to know if it's the same on others.

-- 
tjaden@xxxxxxxxxx - http://www.psynet.net/tjaden/
MOTSPUR (MOT sper), n.  The pesky fourth wheel on a shopping cart that
refuses to cooperate with the others.  -- Rich Hall, "Sniglets"

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