CURLOPT_SHARE(3) on macOS

The text 64f9af09e78c, 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 share handle to use

CURLOPT_SHARE(3) on macOS now