Modbus feature 639 v1.7

This pull request adds support for Modbus protocol (rule keyword : modbus). It is an update of the Modbus feature 639 v1.6 PR to fix Cppcheck issues. Some comment has been updated.

Modbus Request and Modbus Response parsers have been rearranged in order to avoid a memory leak (tx) and unittests have been added to check it.