compositor: implement wayland socket handover (#6930)

* compositor: implement wayland socket handover

This commit implements the compositor side of the Wayland socket
handover protocol as described in the [KDE Wiki]. The CLI options are
chosen so that they are compatible with Kwin.

[KDE Wiki]: https://invent.kde.org/plasma/kwin/-/wikis/Restarting

* main: verify that --wayland-fd is a valid file descriptor

* main: fail if only one of --socket and --wayland-fd is passed