cursor-tracker: Kill unused variable
xwayland: Use %m instead of explicit strerror
xwayland: Put the filename in the error message
xwayland: Clean up display finding code

Split out and make it more manageable.
xwayland: Rename sp to something a bit more handy

We also want to create a socketpair for -displayfd instead
of using signals.
xwayland: Stop using UNIX signals for readiness

The latest Xorg / Xwayland has support for -displayfd being used
in conjunction with an explicit display number. Use that to know
when the X server is ready, rather than UNIX signals, because
they're UNIX signals.
xwayland: Remove some unused includes