Reduce the size of Packet structure

Share memory space for IPV4Vars and (IPV6Vars, IPV6ExtHdrs), since a
packet can only be either IPv4 or IPv6, but not both.

Share memory for TCPVars, UDPVars, ICMPV4Vars and ICMPV6Vars, since a
packet can only be only of these.

Then move other structure members around to remove holes reported by pahole.

This reduces the size of the Packet structure from 2944 bytes (46 cachelines)
down to 1976 (31 cachelines), a 33% reduction.