Issue Description:

I am trying to get libdispatch working on FreeBSD but I am hitting a wall: I cannot get my head around how the runloop implementation dispatches a event to the kqueue once a new job is submitted. So far I was able to get the library to fully compile but most tests are timing out due to a issue getting the kqueue event handler to awake when the poke method is called.

Looking at the Linux implementation (https://github.com/apple/swift-corelibs-libdispatch/blob/master/src/queue.c#L6119) it appears to be creating a new eventfd descriptor and calls eventfd_write on the poke method. A eventfd_read call is only made in the event_epoll.c implementation file BUT I am unable to find where the specific fd (created in queue.c) is forwarded to the epool handler. In other words, how is the event queue implementation informed of that file descriptor?

On the Darwin side of things, it appears to use a mix of kqueue (on the event handling side) and mach ports (on the signaling side), but i still cannot understand how the event_kevent.c implementation becomes aware of that port allocated on queue.c.

In order do get dispatch working on FreeBSD I need to be able to understand the flow that happens between _dispatch_runloop_queue_class_poke * and a event getting fired in the kqueue implementation (which I believe should be happening in _dispatch_kq_poll*).

EDIT: typos