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

Merge branch '4884-use-cpuset-to-get-number-of-cpus' into 'main'

Closes #4884

See merge request isc-projects/bind9!9398