Prepare network statistics structures for migration to PCPU counters.
Use uint64_t as type for all fields of structures.

Changed structures: ahstat, arpstat, espstat, icmp6_ifstat, icmp6stat,
in6_ifstat, ip6stat, ipcompstat, ipipstat, ipsecstat, mrt6stat, mrtstat,
pfkeystat, pim6stat, pimstat, rip6stat, udpstat.

Discussed with:	arch@
Add several macros to help migrate statistics structures to PCPU counters.
Use new macros to implement ipstat and tcpstat using PCPU counters.
Change interface of kread_counters() similar ot kread() in the netstat(1).
Migrate structs arpstat, icmpstat, mrtstat, pimstat and udpstat to PCPU
counters.
Migrate structs ip6stat, icmp6stat and rip6stat to PCPU counters.
Migrate structs in6_ifstat and icmp6_ifstat to PCPU counters.
Migrate struct carpstats to PCPU counters.
Migrate structs ahstat, espstat, ipcompstat, ipipstat, pfkeystat,
ipsec4stat, ipsec6stat to PCPU counters.
Bump __FreeBSD_version to reflect changes in statistics structures.