libbsd: Add a basic kqueue implementation.

It only supports file descriptors and processes (threads),
and a few flags (not all) to go with them.

This has been tested extensively against libuv.

Change-Id: I6fc5930fa7273698172c9c695965842b5df44f03
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6746
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>