#30 modbus demo for RTU

[Jiri Baum]

The modbus demo matplc.conf contains network
definitions for all three modbus types (RTU, ASCII and
TCP), but not all of them are mapped. It's also
somewhat confusing. One of two things should probably
be done:

- make the demo work for all three types (by suitable
duplication of node and/or map entries), or

- make the config for one of the types only, and supply
alternate configs for the other two types.

The second variant would probably be better, as it
would lead to less confusion and give people a much
cleaner starting point for their own configs.