public int
verify(Message m, byte [] b, TSIGRecord old) {
	return verify(m, b, b.length, old);
}
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
src/main/java/core/org/xbill/DNS/TSIG.java