#45 modbus reg_type synonyms

[Jiri Baum]

In modbus master, the reg_type is specified as in_bit |
out_bit | in_word | out_word. While this makes perfect
sense to us programmers, I suspect it's not
particularly user-friendly.

Suggested solution: get the modbus master to accept
various synonyms for these; for instance, in_bit might
have the synonyms: discrete_input, DI, and possibly
even 1x or 1; similarly out_bit (coil, discrete_output,
C, DO, 0x, 0), in_word (input_register, IR, 3x, 3),
out_word (holding_register, output_register, HR, OR,
4x, 4).

The examples and documentation should then be
re-written to use DI, C, IR and HR as the preferred types.