libweston: set the seat automatically based on the XDG_SEAT environment variable

This will allow the seat to be set by the environment as pam_systemd typically
sets the XDG_SEAT variable

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>