[kwin_wrapper] pass the wayland socket name via arguments rather than the env

The initial goal of kwin_wrapper was that it would set up all relevant
environment variables that will be used by the clients of kwin.

This meant having to cache old variables before we overrode them and
pass additional settings to the backends for the old wayland_display. It
works, but with X11 about to move too ends up being unnecessarily
complex. In hindsight it's easier for kwin to have the environment
represent the platform it is currently on, and have kwin explicitly set
variables in the QProcessEnvironment of the session it forks itnto.

This patch is changed so that we set the wayland socket name used by the
wrapper explicitly which is then used by the process environment kwin
uses for the main session.