kqueue, kevent, closeonexec, setitimer, with sysctl and fcntl helpers.

TODO:arm,arm64

Change-Id: I9386f377186d6ac7cb99064c524a67e0c8282eba
Reviewed-on: https://go-review.googlesource.com/118561Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>