Make clpreauth flags function optional

Make clpreauth flags function optional

With one exception (KRB5_PADATA_PKINIT_KX), every padata type
processed by a clpreauth module is now a real preauthentication type.
Reduce the amount of boilerplate required for a clpreauth module by
making the flags method optional if all of the preauth types
advertised by the module are real.