CURLOPT_IOCTLFUNCTION(3) on macOS

The text 547224d3fc51, 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.

callback for I/O operations

CURLOPT_IOCTLFUNCTION(3) on macOS now