nflog fixes

Fixes start up failure if max-size isn't set, so the default is picked.
Adds timeout to the socket, so if we have no traffic we can still shut down Suricata.

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/315
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/231