Use libmodbus to decode incoming Modbus TCP messages

[Abestanis]

This allows us to better handle multiple Modbus ADUs in a single TCP frame (fixes #253).

libmodbus is better at parsing ADUs and we already use it when we are the Modbus client.

I tested this by sending many commands in a short amount of time and verifying via Wireshark that some of them were sent in one TCP frame. I made sure that OpenPLC was sending one response per request, even for those in one TCP packet.

In addition to the receiving problem, I also noticed that the result of write was ignored, which returns the number of bytes written. If we generate a lot of response messages, we could fill up the kernel buffer and drop some bytes. The new approach calls write in a loop until all bytes have been written or write returns an error.