Add mmap_anonymous function

[newpavlov]

Closes #2126

[SteveLauC]

CI failure seems to be unrelated to this PR

---

Probably two change logs need to be added:

### Changed
- `mmap()` no longer takes optional file descriptor as there is a dedicated interface `mmap_anonymous()` for that
   ([#2127](https://github.com/nix-rust/nix/pull/2127))

### Added
- A dedicated interface `mmap_anonymous()` for anonymous mmap has been added
   ([#2127](https://github.com/nix-rust/nix/pull/2127))

[asomers]

The CI failure is fixed on master. You need to rebase.

[SteveLauC]

Code LGTM, still, change logs are needed

[SteveLauC]

Friendly ping @newpavlov, could you please add change logs for this PR? We have changed the CHANGELOG mode in #2149, please see CONTRIBUTING.md on how to add change logs

[newpavlov]

@SteveLauC
Done. I've reverted the mmap changes since they are breaking and additional changelog note (I am not sure if two changelog notes can be added for one PR). I think it's worth to eventually replace Option<F> with just F, but it can wait until next breaking release.

[SteveLauC]

Done.

Thanks for doing this!

I've reverted the mmap changes since they are breaking

Breaking changes are allowed as we will bump our minor version number in the next release

and additional changelog note (I am not sure if two changelog notes can be added for one PR).

I just tested that we can have two change logs with the same PR number in the current mode, so go ahead:) I will document this in CONTRIBUTING.md tomorrow