Protocol:

	-> ASN.1 encoded V5 ticket
	<- int status_code, [int kvno, encode_v4tkt encoded KTEXT_ST]

kvno and V4 ticket are only included if status_code is zero.

The kvno for the converted ticket is sent explicitly because the field
is ASN.1 encoded in the krb5_creds structure; the client would have to
decode (but not decrypt) the entire krb5_ticket structure to get it,
which would be inefficient.
