Implement {Tcp,Unix}Acceptor::{clone, close_accept}

If a task is spinning in an accept loop, there is currently no method of gracefully shutting it down. This PR introduces a way to do so by cloning the acceptor and implementing a close_accept method to unblocking any pending acceptor.

As with other I/O methods like this, it is `#[experimental]` from the start and sadly carries with it a good deal of code to support it. Much of the complication is from the fact that you can now concurrently accept on the same socket.

I tried to add a good deal of tests for this change, but another set of eyes is always appreciated!
> This deadline doesn't take into account any time spent in the earlier libc::accept()

The deadline is an absolute time, so how would it take into account time spent in accept?

> That really needs to be adjusted to use something like time::precise_time_ns() instead.

I'll leave that for another patch?

> Also, if the deadline elapses here, it's just going to end up looping again

Can you explain? I am under the impression that `await` will return an error for the timeout, causing `try!` to return from the function.