Peter M. Groen / libmodbus

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Signed-off-by: Michael Heimpold <mhei@heimpold.de>

Thanks to Marjan Tomas

Thank you AlexMaz.

Thanks to Jan Kardell.

- change return argument from void to int
- update documentation

Thanks Giampaolo Bellini.

Original patch provided by Michael Haberler <git@mah.priv.at>.

Original patch for new connect function by Thomas Stalder.

- export new symbols to create specific behavior (RTU)
- the flag can't only be set by slaves
- new tests

Warning, modbus_receive returns 0 when a query is ignored.

- function removed from TCP and RTU code and from backend structure
- updated documentation
- updated examples

The serial mode and rts flags are now initialized in new function.
Original patch provided by Jaime Alberto Silva
<jaime@sgautomatizacion.com>.

- new function _modbus_rtu_pre_check_confirmation
- new special test value to trigger an invalid response
- add unit test to cover invalid TID too

Original patch provided by Thomas Stalder.

- avoid export of _modbus_rtu_set_rts and change its name to avoid
  confusion
- namespace RTU functions and constants
- use_rts is renamed rts
- add missing '_' in function name
- remove useless debug message in usual write
- wrong message about 'not supported'
- add check for TIOCM_RTS support
- block RTS flow control when not available

bit is used HIGH to able to transmit, or LOW.