Autoscale vm_pageout worker threads from r364129 with CPU count. The
default is arbitrarily chosen to be 16 CPUs per worker thread, but can
be adjusted with the vm.pageout_cpus_per_thread tunable. The
vm.pageout_threads_per_domain tunable is removed to avoid confusion.
There will never be less than 1 thread per NUMA domain, and the previous
arbitrary upper limit (at most ncpus/2 threads per NUMA domain) is