curl_multi_fdset(3) on macOS

The text f4a0c60e2bb0, as macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

extracts file descriptor information from a multi handle

curl_multi_fdset(3) on macOS now