The option names are a little bit confusing, because it's not clear
which ones toggle libseat features, and which ones toggle seatd
features.
Add a "libseat-" prefix to libseat-specific features, to make it
more obvious that they only are about the library.
Allow package maintainers to explicitly select a logind provider
by passing -Dlogind=systemd or -Dlogind=elogind. In case both are
available (e.g. for distributions which support both), this makes
it possible to gte deterministic behavior.
By default, auto-detect the logind provider. That way, users which
have systemd or elogind installed get the backend built by default.
