FreeBSD uses a separate kqueue filter type for lio_listio. This change
adds support for that filter type. Full functionality will be provided
by the mio-aio and tokio-file crates.
* Add PollEvented::into_inner
Consumes a PollEvented and returns its inner io object. Useful for io
types that have exclusive ownership of a resource.
See also https://github.com/tokio-rs/tokio-core/commit/9400ffb