New protocol decoder: Modbus

Yes, it's that time of the week again — libsigrokdecode supports yet another protocol decoder since somewhat recently: modbus.

In the current state the PD stacks on top of the UART decoder and decodes the Modbus RTU protocol. Support for e.g. Modbus ASCII may be added later (to the same PD), possibly also Modbus TCP or other variants (as an extra decoder).

Check the PD's wiki page or Wikipedia for some more info on Modbus in general.

As always, we have a few example files in the sigrok-dumps repo and some test-cases in the sigrok-test repo.

The decoder was contributed by Bart de Waal, thanks a lot!