Nested backends should not use the default Wayland socket?

For background, see: https://gitlab.gnome.org/GNOME/gtk/issues/1741

When Weston runs nested, be that on X11 or Wayland, it is by definition not the main display server. Therefore it should pick a Wayland socket name other than wayland-0 which is the default socket libwayland-client will connect to when WAYLAND_DISPLAY environment variable is not set.

Only the user's default/main compositor should use the default socket.

This will prevent toolkits from mistakenly connecting to the nested Weston by default.

I guess it would be nice to do the same when running with the headless backend, just in case someone runs it outside of the test suite.

What about RDP?

Do we need a new Weston option for "use default socket for listening", whose default value depends on the backend? Hmm, maybe not, there is already -S command line option, and the choice of backend could affect the default socket name.