call sd_seat_can_tty to determine if ttys should be used, instead of hard-coding seat0

Summary: All TTYs are on seat0, but not all seat0s have TTYs

Reviewers: efl, devilhorns

Reviewed By: efl, devilhorns

Subscribers: bu5hm4n, stefan_schmidt, raster, cedric, reviewers, committers

Tags: efl

Reviewed-by: Christopher Michael <devilhorns@comcast.net>
Differential Revision: https://phab.enlightenment.org/D12143