This will help to better determine the number of worker processes to
create in certain situations.
Unlike sysconf(_SC_NPROCESSORS_ONLN) this takes into account per-process
cpu allowed masks as set by sched_setaffinity(2)/cpusets etc.
So while a system may have 64 on-line cpu's, nginx itself may be limited
to using just four of them in which case we should create just four
worker processes not 64 under 'worker_processes auto;'
sched_getaffinity(2) is available on at least Linux & FreeBSD.
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