Define CURVE25519_NWAY::Kerberos.

This module inherits CURVE25519_NWAY and overrides curve25519_final()
to calculate Kerberos keys from the generated PRK.  It defines a calling
convention for $priv which is followed by the other modules which inherit
it: Krb5Admin::KerberosDB and Krb5Admin::Krb5Host::Local.