gh-109649: Add os.process_cpu_count() function (#109907)

* Refactor os_sched_getaffinity_impl(): move variable definitions to
  their first assignment.
* Fix test_posix.test_sched_getaffinity(): restore the old CPU mask
  when the test completes!
* Doc: Specify that os.cpu_count() counts *logicial* CPUs.
* Doc: Specify that os.sched_getaffinity(0) is related to the calling
  thread.