Returns AUTH_OK if the message is authenticated successfully. If
    it returns AUTH_OK, the routine also does the following:
    o  Sets rqst->rq_xprt->verf to the appropriate response verifier.
    o  Sets rqst->rq_client_cred to the "cooked" form of the
       credentials.

    The expression rqst->rq_xprt->verf must be preallocated and its
    length must be set appropriately.

    The program still owns and is responsible for msg->u.cmb.cred and
    msg->u.cmb.verf. The authentication system retains ownership of
    rqst->rq_client_cred, the "cooked" credentials.