Make use of sysconf(3) _SC_NPROCESSORS_ONLN and _SC_NPROCESSORS_CONF

Make use of _SC_NPROCESSORS_ONLN if it exists and fallback to
_SC_NPROCESSORS_CONF for really old operating systems. This adds
support for retrieving the number of CPUs on a few OS's such as
NetBSD, DragonFly and a few others.