Use BSD libc API getifaddrs(3) instead of using kvm(3) to obtain
interface byte counters relying on unstable kernel ABI, which
differs a lot in different BSD operating systems and is very
likely to change in the future. Actually this commit was driven
by the fact that xosview doesn't compile on FreeBSD 11. The change
also simplifies code a lot.

The change was tested on FreeBSD 11, but should work on all *BSD
versions, according to documentation on getifaddrs(3).
In modern (all supported) FreeBSD versions, the vmmeter went to a
per-CPU private regions, and can't be longer easily read via
kvm(3). Thus, utilize the sysctl(3) API to fetch vmmeter data.
Strip empty fields from the nlist.