This changes the PPPoE parser so that it doesn't forward extra bytes
that might be appended after the payload. Instead, it raises a weird if
the payload size doesn't match the size indicated by the header.
This is in line with what other protocol parsers (like UDP) are doing.
Two tests needed to be updated - with this change, the traffic in
pppoe-over-qinq.pcap is now valid TLS. A new trace was introduced for
the confirmation-violation-info test.
Addresses GH-4602
