Optionally ignore modbus packet size restrictions, use TCP packet length

For our in-house Modbus TCP implementations we ignore the silly 260 byte / 125 register limitation on modbus packet sizes. The "quantity of registers" field is two bytes and so can theoretically support up top 2^16 registers, and we allow that. For response packets we ignore the modbus packet length field and instead look at the TCP segment length to determine packet size.