pcap.lua: Added write_record_header(), removed "extra" hack.

The "extra" hack was to hide additional information in the pcap record
by using a capture length larger than the actual packet length. It was
from a very early version of Snabb Switch that generated rich tcpdump
traces for testing-by-postprocessing.
apps/pcap: Now have PcapReader and PcapWriter apps.

These replace the Pcap app, which was a reader.
loadgen: s/Pcap/PcapReader/
ipv6 selftest: Generate selftest-output.pcap.

Contents look pretty sane after a couple of small fixes.