CURLINFO_PRETRANSFER_TIME(3) on macOS

The text 80c40dc67e14, 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.

get the time until the file transfer start

CURLINFO_PRETRANSFER_TIME(3) on macOS now