Share Packet checksum values for TCP, UDP, IPv4, IPv6. ICMPv4 and ICMPv6

Use one structure member in Packet_ to store the checksum for the Packet
instead of 6 different members, only one of which is used at a time.

https://buildbot.suricata-ids.org/builders/ken-tilera/builds/44

The is needed before sharing the TCPvar, UDPvar and ICMPvar members in Packet_.