MPLS Decoder, v4

Better payload guess, don't blindly fall back to ethernet as ethernet should be preceded by a pseudowire.

Raise an alert in the case of unknown payload.

- PR build: https://buildbot.openinfosecfoundation.org/builders/jasonish/builds/13
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/jasonish-pcap/builds/13