runtime: make NumCPU respect zone CPU cap on illumos
On illumos systems, check for the "zone.cpu-cap" resource control when
determining how many usable CPUs are available. If the resource control
is not set, or we are unable to read it, ignore the failure and return
the value we used to return; i.e., the CPU count from
sysconf(_SC_NPROCESSORS_ONLN).