CURLOPT_STDERR(3) on macOS

The text ed786b87de11, 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.

redirect stderr to another stream

CURLOPT_STDERR(3) on macOS now