Checksum validation update v1.2

Version updated following discussion. The counter of invalid packets is now a standard counter.

This PR also features a small cleaning (last patch).

PR build: https://buildbot.suricata-ids.org/builders/regit/builds/60

Tickets: https://redmine.openinfosecfoundation.org/issues/751 and https://redmine.openinfosecfoundation.org/issues/752