`Event::fflags()` accessor missing for `EVFILT_READ`/`EVFILT_WRITE` kqueue events

The `event::kqueue::Event` struct does not expose the `fflags` field of the underlying `kevent` for `EVFILT_READ` and `EVFILT_WRITE` events. That field is currently only accessible indirectly through the filter() method for `EVFILT_VNODE`, `EVFILT_PROC` and `EVFILT_USER`.

Per the kqueue(2) man page on macOS/FreeBSD:
> If the read direction of the socket has shutdown, then the filter also sets EV_EOF in flags, and returns the socket error (if any) in fflags.

So it is kinda important to have access to that. There are workarounds of course, but an accessor would be cleaner:

```rust
impl Event {
/// Get the filter-specific flags for this event.
///
/// For `EVFILT_READ`/`EVFILT_WRITE` on sockets, this contains the socket
/// error (e.g. `ECONNRESET`) when `EV_EOF` is set in `flags()`.
pub fn fflags(&self) -> u32 {
self.inner.fflags
}
}
```