UTIL: Introduce --socket-activated cmdline option for responders
    
    This option is going to be used for socket-activated responders as they
    are special in some ways. So, by knowing this option we can:
    - skip owning the debug files as it will be done by the responders' unit
    files;
    - skip "become_user()", as the process will already be started by the
    proper user bu the responders' unit files;
    - setup a timeout to shutdown the responder in case it has been
    socket-activated and is idle.
    
    This option has been encapsulated on is_socket_activated() function,
    which will always return "false" when called on platforms where systemd
    is not supported.
    
    For 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>