By default, configure will now look for libmodbus (installed from a
separate deb, provided by Ubuntu for Precise and by the linuxcnc.org
debian archive for Hardy and Lucid) and error out if it can't be found.

The user can disable this check (and disable compilation of all drivers
that use libmodbus) by configuring with "--without-libmodbus".

- Arduino example added.
  Tested with Arduino Mega 2560 R3 using Modbusino over USB at 115200 bps.
- New parameters in config file.
  HAL_MODULE_NAME, MB_RESPONSE_TIMEOUT_MS and MB_BYTE_TIMEOUT_MS.
- New HAL pin "num_errors" for each transaction. num_errors = 0 if last
  transaction OK, else the count of failed transactions.
- Fix a bug, OPTIONAL's parameters may be 0 instead of default value if
  not specified in 1st transaction.

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

- Mb2hal now uses libmodbus3 instead of the old libmodbus file
  located in src/hal/user_comps/modbus.c used by gs2_vfd.c
  # Also fix a TIMEOUT bug (due to this unpatched old libmodbus file).
- Better INI file documentation (HOW-TO).
- Better Submakefile.
- Source code cleaned and reordered.
  # Now is posible to add new functionallity.

See LogBook.txt for more information.

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

Nothing big, two instances of dead initialization (so, a variable is
explicitly set at define-time, but then written to again before being read).

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>

This reverts commit ca835b429478324639df779e26005ca43b8917d1.

Victor says it is needed.

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>