Completely disentangle xpdev/xpbeep.c from conio/sdlfuncs.c

SDL audio should work from any thread, and on the platforms that care,
SDL video is coddled appropriately.
The only question is around SDL_Quit() in an audio-only build, but we
can punt on that since the default builds will never do that... the only
thing I'm aware of that could be broken that way is tone, which nobody
likes anyway.