Kenny Levinsen authored Oct 21, 2020 and Pekka Paalanen committed Apr 14, 2021

This adds support for libseat as a seat backend. libseat provides seatd,
(e)logind and direct seat backends as compile-time and runtime options.

The backend is currently disabled by default. It can be enabled through the
launcher-libseat option.

Signed-off-by: Kenny Levinsen <kl@kl.wtf>