Remove pkt variable from Packet structure.

The uint8_t *pkt in the Packet structure always points to the memory
immediately following the Packet structure. It is better to simply
calculate that value every time than store the 8 byte pointer.
Fix pfring so that zero-copy mode can work.

Detect when default_packet_size is zero, which enables zero-copy mode for
pfring and in that case, do what AF Packet does and set pkt_ext pointer to
the data and set PKT_ZERO_COPY flag.