compositor-fbdev: support the --seat option, (and XDG_SEAT variable)

This allows the fbdev backend to run on, and use devices from the
specified seat, similar to the drm backend.

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