Add fs.NotifyOnOOM for out-of-memory notifications

See dotcloud/docker#6212 for more discussion.
I don't believe there are any errors that can occur here; a `read` on an eventfd should only fail if the buffer is less than 8 bytes. As for waiting callers... yes. I've assumed that callers are observing other things about the container / cgroup, and shut down appropriately.

Ideally such a notification API would also support canceling notifications; but this brings along the complexity of introducing epoll, and I (at least) haven't yet needed it.