Add kqueue(2) support to the WaitEventSet API.

Add kqueue(2) support to the WaitEventSet API.

Use kevent(2) to wait for events on the BSD family of operating
systems and macOS.  This is similar to the epoll(2) support added
for Linux by commit 98a64d0bd.

Author: Thomas Munro
Reviewed-by: Andres Freund, Marko Tiikkaja, Tom Lane
Tested-by: Mateusz Guzik, Matteo Beccati, Keith Fiske, Heikki Linnakangas, Michael Paquier, Peter Eisentraut, Rui DeSousa, Tom Lane, Mark Wong
Discussion: https://postgr.es/m/CAEepm%3D37oF84-iXDTQ9MrGjENwVGds%2B5zTr38ca73kWR7ez_tA%40mail.gmail.com