CURLOPT_PRIVATE(3) on macOS

The text c7f65b1f988b, 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.

store a private pointer

CURLOPT_PRIVATE(3) on macOS now