curl_easy_perform(3) on macOS

The text 7886754e2c6c, 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.

perform a blocking file transfer

curl_easy_perform(3) on macOS now