Adds a CPU count fact, that can be used to determine how many CPUs are
presently on the machine and ready for use (online). We get this by
reading from a netlink socket to the kernel, and the kernel sends us
uevents when CPUs are added, removed, and brought online or offline.
Whenever one of these events are received, we look in sysfs to update
the fact's Stream with the number of online CPUs.