Add `AsRawFd ` for `nix::dir::OwningIter`

[deantvv]

OwningIter is just a wrapper around Dir which implements AsRawFd which makes this easy (or even trivial) to implement.

The only thing that I could think of this may be unsound is after calling drop() on Dir will make the RawFd invalid but this is the same as AsRawFd for Dir which will also suffer from this issue. Link is the discussion of AsRawFd in stdlib.

I can implement this if this feature is approved.

[deantvv]

Close since it is already merge in #1563