mb2hal: HAL component to communicate with Modbus devices.

Userspace HAL component to communicate with one or
more Modbus devices.

Tests:
- TCP some functions = Remote IO Delta RTU-EN01,
  24 inputs, 8 outputs.
- RS232 some functions = Servo Drive Delta ASDA-B (1021-A 1Kw).
- TCP all functions = MoD_RsSim V8.20 (www.plcsimulator.org)
  on Windows XP SP3.

Signed-off-by: Michael Haberler <git@mah.priv.at>