package/seatd: new package

Introduce a seatd package, which can be used by wlroots 0.12.0 and
newer. The package includes both a library (always built) and an
optional seat management daemon.

The library can use systemd-logind, the seatd daemon, or a simple
builtin in-process mode. Build options are introduced for the daemon
and the built-in mode, as to allow selecting the built-in mode as
default when both the daemon and systemd-logind are not being built.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>