Nflog v2

This PR contains some fixes from the previous:
- indent fixed
- some comments added
- passing the packet to the rest of the engine

Some details:
it seems to work not only in single runmode, but I still got the following error:

[3733] 28/2/2014 -- 04:04:15 - (tm-threads.c:1783) <Error> (TmThreadDisableThreadsWithTMS) -- [ERRCODE: SC_ERR_FATAL(178)] - Engine unable to disable detect thread - "RecvNFLOG".  Killing engine

Previous PR: https://github.com/inliniac/suricata/pull/854