Support for RTU over TCP connection (transport mode not covered by modbus standard but frequently used).

Usefull for scenarios where simple Ethernet-to-Serial converter is used to access modbus network.
In most cases converters (such as Tibbo DS100, Planet ICS-100, GridConnect Net232jr etc) does not convert Modbus-RTU to Modbus-TCP but simply encapsulate RAW serial
port data into TCP stream.
At this moment libmodbus does not support this transport method (it's not standard compliant) but there exists feature request #192
(https://github.com/stephane/libmodbus/issues/192) and patch (https://groups.google.com/forum/#!msg/libmodbus/xm8S6MMDhgM/Qd97y9_mhwgJ) that addresses this issue.
Patch for version 3.0.6 of libmodbus attached in file libmodbus-3.0.6_rtu-over-tcp.patch
Patch adding support of RTU-over-TCP connection to libmodbus 3.0.6.