Changed signature for openat function

[carlosb1]

Fixing #1850 , Changed signature to use an Optional rawfd

[SteveLauC]

PR LGTM, except for the missing change log.

You need to add a change log to CHANGELOG.md, it would be something like this:

## [Unreleased] - ReleaseDate

### Changed

- xxx

- `openat()` and `Dir::openat()` now take optional `dirfd`s
  ([#2134](https://github.com/nix-rust/nix/pull/2139))

And, it would be great if you squash your commits into a single one :)

---

Update: commits don't need to be squashed as this will be done automatically

[carlosb1]

I updated the changelog as you commented. I expect now it is correct

[carlosb1]

I can not merge it, I don't have write access, very happy to contribute in anything else.

[SteveLauC]

Thanks for doing this! :)