CURLOPT_SSLKEYTYPE(3) on macOS

The text 6b5ee1388211, 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.

set type of the private key file

CURLOPT_SSLKEYTYPE(3) on macOS now