user/x11drv: implement (Un)RegisterHotKey

[Vincent Povirk]

This is based on Robert Reif's patch. The tests are the same I think.

Come to think of it, it would probably have worked if I sent different
parts of the diff separately earlier. But now that I've screwed that
up once (and the introductory screw-up that I do with EVERY SINGLE
GROUP I INTRODUCE MYSELF TO EVEN THOUGH I SERIOUSLY DO NOT TRY is
over), I'm not trying it again.

-- 
Vincent Povirk