Add skeleton for Modbus driver.
Update .gitignore to include binaries.
modbus: run Mosquito loop in separate thread.
modbus: initial libmodbus integration, somewhat flaky at the moment.
modbus: made it a bit less flaky (flush after init, recovery).
modbus: fix input register handling.
modbus: made it possible to specify control types for Modbus registers.
modbus: made poll interval & debug mode configurable.
modbus: updated config for MSU34-TLP.
modbus: scaling support; fix channel type handling.
modbus: don't use 'double' for non-scaled values (including coils / discrete inputs).
modbus: added tM-P3R3 registers.
modbus: hierarchical config support.
modbus: get rid of unneeded data copying in the MQTT message handler.
modbus: added support for multiple serial ports.
modbus: fixed config example (moved "debug" option up).
modbus: added single port config for PC-based testing.
modbus: use readonly switches for discrete inputs.
modbus: added wo-switch channel type (used for to write-only coils).
modbus: channel format support.

MSU34+TLP uses 8-bit signed format for its temperature values.