CURLOPT_PRIVATE(3) on macOS

The text 7e3af8c82034, as OS X 10.11.6, OS X 10.11.5, OS X 10.11.4, OS X 10.11.3, OS X 10.11.2, OS X 10.11.1, OS X 10.11, OS X 10.10.5 shipped it.

store a private pointer

CURLOPT_PRIVATE(3) on macOS now