UTIL: Introduce --dbus-activated cmd option for responders
    
    Similarly to the --socket-activated cmd option, --dbus-activated cmd
    option is going to be used for dbus-activated responders in order to
    easily setup a timeout to shutdown the idle responder in case it has
    been socket activated and is idle.
    
    This option has been encapsulated on is_dbus_activated() function,
    which will always return "false" when called on platforms where systemd
    is not supported.
    
    For now any of the services are taking advantage of this newly
    introduced option/function, but later on in this series it's going to be
    used.
    
    In order to use it, just add SSSD_RESPONDER_OPTS to the poptOption
    structure.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2243
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>