Implement CPU counts for platforms other than Linux & HP-UX, except Windows

Replaces Linux /proc/stat parsing with sysinfo() call, which returns the same
result but also works on various other platforms. Adds sysctl() call for
BSD-derived platforms.

Documentation update removed Linux-only caveat; clarified virtual vs. physical
CPUs; added mention of hard classes which are set.

Configure.ac update adds the check for BSD sys/sysctl.h to allow for
CPU counting on that platform, and the necessary includes in sysinfo.c.
These updates are written in such a way that they will be easy to merge
with pull request https://github.com/cfengine/core/pull/229 as they
overlap a bit.
Rearrange Windows-not-supported component in GetCPUInfo()

Changed Windows section to a "not supported" notice at the top of the
function, to prevent unnecessary allocation of buffer and count variables
in this function, and to inform that there is no CPU count capability.
Merge branch 'cpu_count'