clib.h: Added memcmp(3).
apps/pcap: Now have PcapReader and PcapWriter apps.

These replace the Pcap app, which was a reader.
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.
vhost: Added missing packet.deref() in reclaim_transmitted_packets().
apps/vhost_apps: Added TapVhost app.

This app creates a Linux tap interface with fast vhost I/O.
intel_app: Partial fix after changes in app interface.