util: Use systemd-login to check user sessions
Use systemd-lgin in preference to check if the user is logged in or not.
Fall back to the old method if no systemd-login support is available at compile
time or if it returns a fatal error, and can't determine the status of the user
on its own.
This will allow to consider a user really active (in order to reuse or refresh
crdentials) only if it really is logged into the system, and not just if one
of the user's processes is stuck around.
Resolves:
https://fedorahosted.org/sssd/ticket/2084