CURLOPT_SEEKFUNCTION(3) on macOS

The text f665269356fe, 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.

user callback for seeking in input stream

CURLOPT_SEEKFUNCTION(3) on macOS now