o Minor refactoring:
    - Refactor count_usable_descriptors to use named enums for exit_only.
      count_usable_descriptors now uses named exit_only values:
       *  USABLE_DESCRIPTOR_ALL
       *  USABLE_DESCRIPTOR_EXIT_ONLY
    - Add debug logging code for descriptor counts.
      This resolves nickm's request in bug 13718 to improve argument
      readability.