x11: use xrandr to retrieve display refresh rate

Drop use of the ancient XF86VM, and use the slightly less ancient Xrandr
extension to retrieve the refresh rate. Xrandr has the advantage that it
supports multiple monitors (at least the modern version of it).

For now, we don't attempt any dynamic reconfiguration. We don't request
and listen to Xrandr events, and we don't notify the VO code of changes
in the refresh rate. (The later works by assuming that X coordinates map
directly to Xrandr coordinates, which probably is wrong with compositing
window manager, at least if these use complicated transformations. But I
know of no API to handle this.)

It would be nice to drop use of the Xinerama extension too, but
unfortunately, at least one EWMH feature uses Xinerama screen numbers,
and I don't know how that maps to Xrandr outputs.
build: drop check for XF86keysym.h

This is always included in the Xorg development headers. Strictly
speaking it's not necessarily available with other X implementations,
but these are hopefully all dead.
build: use pkg-config for xscreensaver