curl_global_init_mem(3) on macOS

The text df9c7d7fc552, 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.

Global libcurl initialisation with memory callbacks

curl_global_init_mem(3) on macOS now