vlan/QinQ: add vlan_qinq counter

This patch introduces a new counter "decoder.vlan_qinq". It counts
packets that have more than two stacked vlan layers.

Packets with 2 vlan layers will both increment "decoder.vlan" and
"decoder.vlan_qinq".

https://redmine.openinfosecfoundation.org/issues/1102

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/189
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/109