When importing a Kerberos name for GSS pre-auth, first try to import the name
as GSS_KRB5_NT_PRINCIPAL_NAME. If that fails, fall back to GSS_C_NT_USER_NAME.
The target (acceptor) name for GSS-API pre-authentication should be the name of
the TGS, not the server name in the AS-REQ, as it is the KDC which is being
mutually authenticated. If the client is not requesting a TGT, they may differ.
Add support for GSS-API pre-authentication to the KDC, using a simplified
variation of draft-perez-krb-wg-gss-preauth-02 that encodes GSS-API context
tokens directly in PADATA, and uses FX-COOKIE for state management.
More information on the protocol and implementation may be found in
lib/gssapi/preauth/README.md.
