libcurl-multi(3) on macOS

The text edda6752a09d, as OS X 10.11.6, OS X 10.11.5, OS X 10.11.4, OS X 10.11.3, OS X 10.11.2, OS X 10.11.1, OS X 10.11, OS X 10.10.5 shipped it.

how to use the multi interface

libcurl-multi(3) on macOS now