Remove pkt 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.

Passes all regression tests:
https://buildbot.suricata-ids.org/builders/ken-tilera-pcap/builds/13
https://buildbot.suricata-ids.org/builders/ken-tilera/builds/76

This replaces pull request: https://github.com/inliniac/suricata/pull/617 and is simply a rebasing to the current master.

@regit approved that pull request.