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 98a64d0b.

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