SYSDB: Return ENOENT for mpg with local provider
    
    We need to return ENOENT for local provider if user have
    just magic private group. Otherwise we would not be able
    to detect such situation in sss_groupshow
    
        /* The search itself */
        ret = group_show(tctx, tctx->sysdb,
                         tctx->local, pc_recursive,
                          tctx->octx->name, &root);
        /* Also show MPGs */
        if (ret == ENOENT) {
            ret = group_show_mpg(tctx, tctx->local,
                                 tctx->octx->name, &root);
        }
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3644
    
    Merges: https://pagure.io/SSSD/sssd/pull-request/3720
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>