Add Modbus RTU Server

[stefanportmann]

Added the RTU server in the same manner as the TCP server. You can use the same modbus.conenctionHandler.
You can start the server like this:

func ModbusServeRTU(ttypath string, modbusAddress uint8) (err error) {
	var server *modbus.ModbusRtuServer

	go modbusConnectionHandler.StartUptimeCounting()

	server, err = modbus.NewRTUServer(&modbus.ModbusRtuServerConfig{
		TTYPath:       ttypath,
		ModbusAddress: modbusAddress,
	}, modbusConnectionHandler)
	if err != nil {
		modbusLog.Printf("Failed to create RTU server: %v\n", err)
		return
	}

	err = server.Start()
	if err != nil {
		modbusLog.Printf("Failed to start RTU server: %v\n", err)
		return
	}
	modbusLog.Printf("RTU server started on %v, with address %v\n", ttypath, modbusAddress)
	return
}