ch04: Implement kqueue poll for macOS/BSD

[codyrioux]

This pull request contains a more or less 1:1 translation of the epoll code in ch04/a-epoll to kqueue eliminating the need for macOS and BSD users to run a virtual machine.

My objective in writing this was to motivate myself to understand the content of chapter 4 deeply enough to be capable of performing a translation. With that said I didn't know much about epoll before reading this chapter and knew nothing about kqueue until 4 hours ago, so I'm happy to take any feedback on the topic. However I did go through and attempt to comment and explain the differences between the kqueue code and the epoll code in the book such that a macOS or BSD user could follow the code and the chapter at the same time without a loss of fidelity.

Thank you for writing this book by the way! On occasion you run into a technical book that is just exceeds all your expectations and this is one of them. Well done.

[cfsamson]

Thanks for the encouraging words! Doing what you did here is a great way to learn.

I'm currently bogged down by work and a future second revision of this book, but I’ll get back to this later. Just wanted to let you know I appreciate the contribution and effort even though I wont be able to comment before later.