CURLOPT_DNS_SERVERS(3) on macOS

The text 470c23e64d4f, 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 preferred DNS servers

CURLOPT_DNS_SERVERS(3) on macOS now