SYSTEMD: Avoid starting a responder socket in case SSSD is not started
    
    As systemd adds "Before=sockets.target" to any socket unit by default,
    during the startup of the system we can end up having a responder socket
    up, being contacted while SSSD is shutdown.
    
    By using "DefaultDependencies=no" we ensure that sockets.target won't
    trigger the sockets' startup and that it only will be done when SSSD is
    up. The downside of using "DefaultDependencies=no" is that we have to
    deal with conflicts and add "Conflicts=shutdown.target" to each of the
    sockets unit.
    
    This patch has been suggested by Lukáš Nykrýn.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3298
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>