CPU detection should consider CPU affinity

(Ref: https://techsupport.isc.org/Ticket/Display.html?id=23847)

A customer has stated that the current method of detecting CPU count via sysctlbyname("hw.ncpu" ...) does not consider CPU affinity.

A general suggestion around cpuset_getaffinity() was offered:

#include <sys/cpuset.h>

cpuset_t mask;
CPU_ZERO(&mask);

if (cpuset_getaffinity(CPU_LEVEL_WHICH, CPU_WHICH_TID, -1,
sizeof(mask), &mask) != 0)
...error_handling...;
result = CPU_COUNT(&mask);