Follow the number of CPU set by taskset/cpuset

Follow the number of CPU set by taskset/cpuset

Administrators may wish to constrain the set of cores that BIND 9 runs
on via the 'taskset', 'cpuset' or 'numactl' programs (or equivalent on
other O/S), for example to achieve higher (or more stable) performance
by more closely associating threads with individual NIC rx queues. If
the admin has used taskset, it follows that BIND ought to
automatically use the given number of CPUs rather than the system wide
count.

Co-Authored-By: Ray Bellis <ray@isc.org>