curl_multi_socket(3) on macOS

The text c9c8099fd5f2, 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.

reads/writes available data

curl_multi_socket(3) on macOS now