pcap: Terminate when we're at the end of the file
ppkt: Add pull function.

ppkt_pull lets us skip over headers without having to allocate a new
ppkt_t and copy the remaining data into it.
pcap: Parse IP header.

In order to be able to swap between the tcp.c and the pcap.c layers
freely we need pcap to offer TCP payloads to the layer above it.
analyze: start to use the new pcap layer

Later we'll want to use the tpkt layers and above.