Peter M. Groen / libmodbus

Thank you @pboettch for initial proposal.

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>

It concerns only the files in 'tests' directory.

Thanks to Damian Zieliński.

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

Split the original modbus_receive function in two functions to avoid
the strange -1 value to ignore the sockfd argument.

These functions have no meaning in RTU so it's better to specialize
the names and remove them from the backend.

- remove the functions from the backend
- update tests to handle RTU mode (master and slave)
- add command line options to tests (rtu or tcp)

All private functions and constants are now prefixed by _.
The modbus protocol uses entry points to call specific functions.