curl_easy_duphandle(3) on macOS

The text 5f4d1c96a6d1, 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.

Clone a libcurl session handle

curl_easy_duphandle(3) on macOS now