P11_CHILD_NSS: More restrictive permissions
    
    p11_child_nss runs as root and we must be carefull about security. This
    patch adds more restrictive permissions on it. There is no reason for
    0077, so we use 0177 umask.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2424
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>