getsockopt_wrapper: add support for PAM clients
PAM clients expect that the private socket of the PAM responder is
handled by root. With this patch getsockopt_wrapper can return the
expected UID and GID to PAM clients.
Related to https://pagure.io/SSSD/sssd/issue/3500
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>