Harden the systemd service unit for seatd

Description:

By default, the systemd unit for seatd is provided by upstream but it's insecure and extremely basic. Specifically, it's marked as insecure by systemd-analyze with a value greater than 9, and it should at least be hardened to a similar level as the systemd-logind service file or polkit.

Additional info:

• package version(s): 0.8.0-1