CURLOPT_TCP_KEEPALIVE(3) on macOS

The text 5477f4a1e98d, 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.

enable TCP keep-alive probing

CURLOPT_TCP_KEEPALIVE(3) on macOS now