curl_multi_cleanup(3) on macOS

The text 62bde7ecbc93, as Mac OS X 10.6.8, Mac OS X 10.6.7, Mac OS X 10.6.6, Mac OS X 10.6.5, Mac OS X 10.6.4, Mac OS X 10.6.3 shipped it.

close down a multi session

curl_multi_cleanup(3) on macOS now