[9.18] chg: usr: 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).

If the admin has used taskset, the `named` will now follow to automatically use the given number of CPUs rather than the system wide count.

Closes #4884

Backport of MR !9398

Merge branch 'backport-4884-use-cpuset-to-get-number-of-cpus-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9443