server: Allow absolute paths in WAYLAND_DISPLAY
The compositor should handle absolute paths in WAYLAND_DISPLAY like the clients, ie not adding the XDG_RUNTIME_DIR prefix if it's an absolute path.
This allows to create the wayland socket in a separate directory for system compositors if desired. Clients could then directly inherit the environment variable.
Signed-off-by: Loïc Yhuel loic.yhuel@softathome.com