Auto merge of #146341 - Qelxiros:dirfd-minimum, r=tgross35

minimal dirfd implementation (1/4)

This is the first of four smaller PRs that will eventually be equivalent to rust-lang/rust#139514.

A few notes:

- I renamed `new` to `open` because `open_dir` takes `&self` and opens a subdirectory.
  - I also renamed `open` to `open_file`.
- I'm not sure how to `impl AsRawFd` and friends because the `common` implementation uses `PathBuf`s. How should I proceed here?

The other PRs will be based on this one, so I'll make drafts and mark them ready as their predecessors get merged. They might take a bit though; I've never done this particular thing with git before.

Tracking issue: rust-lang/rust#120426

try-job: aarch64-apple
try-job: dist-various*
try-job: test-various*
try-job: x86_64-msvc-1
try-job: x86_64-mingw*

Author: bors