wayland: Port logind session from GetSessionByPID

It's needed to make kwin_wayland properly systemd managed. A service
launched using systemctl --user is per user, not session.

As a consequence, user services run outside the session. Applications
that deal with user session need some quirks to work with systemd boot.

Unfortunately, GetSessionByPID() doesn't work well with systemd boot,
see bug 433364. GetSession(auto) is shown to work better.