curl_easy_duphandle(3) on macOS

The text be01c9f277cf, as Mac OS X 10.6.8, Mac OS X 10.6.7, Mac OS X 10.6.6, Mac OS X 10.6.5, Mac OS X 10.6.4, Mac OS X 10.6.3 shipped it.

Clone a libcurl session handle

curl_easy_duphandle(3) on macOS now