When debugging Xwayland-related issues, a common first step in debugging
has been to ask the reporter to move their real Xwayland to
/usr/bin/Xwayland.bin, and create a shell script starting Xwayland with
extra arguments under the original /usr/bin/Xwayland location.
Introducing a `WLR_XWAYLAND` environment variable makes this less
invasive, by allowing the user to swap out Xwayland without resorting to
global system changes (or source patches).
The xwayland ready signals are used to do initial setup like starting xwm.
Discarding the signals means that the handler functions will not be called
in the case that Xwayland is restarted and thus, xwm managed clients fail.
Fixes #2174."
