Peter M. Groen / libmodbus

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

This replaces the lengthy license text headers with a short
and standardized license tag. See http://spdx.org for details.

This is useful e.g. for license compliance tools which scan
through files and generate a report of the licenses used
in a project.

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

Thank you Petr Gladkiy and MarjanTomas.
Sorry for the delay...

From Michael Heimpold:
Just a nitpick: the INADDR_* macros are defined in host byte order,
compare INADDR_LOOPBACK. So it would be reasonable to wrap this with
htonl(INADDR_ANY). However, every experienced socket programmer should
know that INADDR_ANY equals 0.0.0.0, passing it through htonl does not
have any effect.

- protect against NULL IP address BTW
- update documentation

- allow an empty string or NULL for node argument
- protect against NULL in service argument
- new test for NULL service
- update documentation

Thanks to Perry Kundert for bug report and initial patches.

- change return argument from void to int
- update documentation

Thanks to Petr Gladkiy

Thanks to oldfaber and endrelovas.

It will better to code something such as:
https://github.com/crossroads-io/libxs/blob/master/src/err.cpp

I don't intend to rewrite glib neither :(

Thanks to oldfaber

Thanks to Thomas Stalder and Torello Querci for the report.

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