Add pcap support to tracebox (for validation purposes)

Tracebox can now write and read packets from pcap. To use the
feature, the option -i must be used as follows
  * -i pcap:<output_pcap_file>:<input_pcap_file>

output_pcap_file must be a regular file or pipe and input_pcap_file
must be a pipe (created with mkfifo).

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>