Correct tortuous comparison that triggers warnings

The code was actually correct but boy, this is no way to write
a comparison.  XineramaQueryVersion() returns zero on failure, so
compare against that.  Not False, not Success, not Maybe.
Plug memory leak in xinerama

If there is only one Xinerama screen we do not care about Xinerama but
we should still free resources we just obtained.
Comment on why XKeycodeToKeysym is still needed.

Prodded by Jeff Abrahamson.