compositor: Set WAYLAND_DISPLAY if we're given a --socket argument
compositor: Set WAYLAND_DISPLAY if we're given a --socket argument

This lets us run multiple compositors (nested or on different VTs).

https://bugs.freedesktop.org/show_bug.cgi?id=46629

Tarnyko/weston-xdg_surface_present,Tarnyko/weston-xdg_surface_present,jonnylamb/weston,sir-murray/weston,xorgy/weston,eyolfson/weston,sir-murray/weston,udoprog/weston,Fantu/compositor-spice,mchalupa/weston,krezovic/weston,udoprog/weston,mchalupa/weston,Gnurou/weston,krezovic/weston,Fantu/compositor-spice,jonnylamb/weston,eyolfson/weston,kwm81/weston,xorgy/weston,Fantu/compositor-spice,giucam/weston,kwm81/weston,giucam/weston,Gnurou/weston