cluster: propagate bind errors

This commit fixes a bug where the cluster module fails to propagate
EADDRINUSE errors.

When a worker starts a (net, http) server, it requests the listen socket
from its master who then creates and binds the socket.

Now, OS X and Windows don't always signal EADDRINUSE from bind() but
instead defer the error until a later syscall. libuv mimics this
behaviour to provide consistent behaviour across platforms but that
means the worker could end up with a socket that is not actually bound
to the requested addresss.

That's why the worker now checks if the socket is bound, raising
EADDRINUSE if that's not the case.

Fixes #2721.