CpuCounter: use count_via_sysctl for all bsd's

FreeBSD, NetBSD, and OpenBSD and all forks that retain "bsd" in the os
name most likely have `sysctl hw.ncpu`, so try count_via_sysctl first
Merge pull request #261 from jcs/patch-1

CpuCounter: use count_via_sysctl for all bsd's