if: Per-cpu ifnet/ifaddr statistics, step 3/3

Make ifnet stats per-cpu.  The per-cpu stats are stored in cache aligned
ifdata_pcpu struct.

- if_mib code is adjusted, so callers of the IFMIB_IFDATA sysctl will not
  aware of the ifnet per-cpu stats, e.g. systat -if
- User space programs that directly access ifnet.if_data are modified to
  aware of the new location of the ifnet stats, e.g. netstat -in
netstat: Make "netstat -w" aware of new ifnet stats location