code: public int
  verify(Message m, byte [] b, TSIGRecord old) {
  	return verify(m, b, b.length, old);
  }
docs: Verifies a TSIG record on an incoming message.  Since this is only called
  in the context where a TSIG is expected to be present, it is an error
  if one is not present.  After calling this routine, Message.isVerified() may
  be called on this message.
  @param m The message
  @param b The message in unparsed form.  This is necessary since TSIG
  signs the message in wire format, and we can't recreate the exact wire
  format (with the same name compression).
  @param old If this message is a response, the TSIG from the request
  @return The result of the verification (as an Rcode)
  @see Rcode