CURLMOPT_MAXCONNECTS(3) on macOS

The text 29e3662de5bb, 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 size of connection cache

CURLMOPT_MAXCONNECTS(3) on macOS now