ResolutionTest, NearestResolution: Handle multiple outputs per screen on Linux.
SetResolution: Make useable on Linux + single-output x-screen at least.

SetResolution can't handle multiple video outputs per screen
in a reasonable manner, as possible on Linux, where one x-screen
can have multiple outputs.

Make it distinguish 1 vs. multi-output setups and provide
proper behavior on single-output screens. On multi-output
screens on Linux, warn user about limited functionality and
do the best it can.