Added support for Modbus RTU (over serial port)

Added support for Modbus RTU (Modbus over serial line) when check_modbus is compiled with libmodbus 3.0.0 or newer.
I'm unable to test TCP/IP connections so I hope nothing's broken (everything looks fine).
Tested over last 10 days reading 12 registers every 1 minute from 2 different devices over RS232 port (+converter).

Update: TCP/IP connections tested briefly (12 registers/2 devices every minute for ~3h) no problems found.