add tcpreplay tool

Closes #515.

Tcpreplay is a tool for replaying network traffic from files saved with
tcpdump or other tools which write pcap(3) files.

The tcpreplay suite also ships with other tools (tcprewrite,
tcpreplay-edit) for manipulating pcap files.

[Peter: minor tweaks]
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>