Fix remote buffer overflow vulnerability (closes #25, #105)

It's strongly recommended to update your libmodbus library if you
use it in a slave/server application in a not trusted environment.

Debian package of libmodbus 3.0.4 already contains a patch to
mitigate the exploit but the patch isn't as strong than this one.
Slight refactor of unit-test-client
Tests on reading 0 or max + 1 registers for function 0x17
Support RTU in new raw requests tests
Updated NEWS file
Bump version to v3.1.1