session: Add strict WAYLAND_DISPLAY validity checks

Enough with not checking we actually have a Wayland compositor around at
all: start requiring the WAYLAND_DISPLAY socket actually exists as an
absolute path or relatively under XDG_RUNTIME_DIR.

Additionally if WAYLAND_DISPLAY isn't an absolute path to the socket
(most setups) ensure XDG_RUNTIME_DIR is set and error with a typically
appropriate message.