CURLOPT_HTTPPOST(3) on macOS

The text 809024e0eeb4, 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.

specify the multipart formpost content

CURLOPT_HTTPPOST(3) on macOS now