modbus: fix conflicts between registers with same indices but different slave addresses.
modbus: support 'enabled' property for ports / devices / channels.
modbus: slightly better error handling.
modbus: publish .../meta/order for channels.

Make channels appear in UI in the same order as they're specified in the
config.
modbus: added support for 'on_value' channel property.

DRB88 may use input registers as discrete inputs with
value of 100 corresponding to 'off' value and value of 101
corresponding to 'on' value. Need to display these as
read-only switches.
modbus: added config section for DRB88 in config-pc.json.