Use BSD libc API to fetch data about interface statistics instead
of nosing in kernel memory. This API should work on all versions
of FreeBSD/NetBSD/OpenBSD/Dragonfly.

This fixes compilation on FreeBSD 11, where kernel structures
have changed and protects against future breakages. It also
make the file much simplier.

Tested by:	Daniel Engberg <daniel.engberg.lists pyret.net>