kevent_qos(2) on macOS

The text 56b9cb233bc2, as macOS 10.12.6, 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.

kernel event notification mechanism

kevent_qos(2) on macOS now