Peter M. Groen / libmodbus

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 Denis Davydov

- more coherent
- namespace
- opaque and smaller context
- usual wording

The library is now simpler to include (only one <modbus.h>)
To avoid confusion MB_VERSION defines have been renamed to
LIBMODBUS_VERSION.

This reverts commit df0cf7927249954f15e0aa85b02d21b990fa9af9.
Use <modbus.h> in tests.

Inspired by the Clutter project and work done by Florian Forster.