dirfd: preliminary unix and windows implementations

[Qelxiros]

Tracking issue: #120426

As per this comment, this issue needs someone to start work on an implementation, so I've implemented a couple functions for UNIX. There's a lot more work to be done here (most of the feature), so I'd love some guidance on what needs fixing in this PR and any notes on how to proceed. Thanks!

[rustbot]

r? @workingjubilee

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[jieyouxu]

r? libs

[bors]

☔ The latest upstream changes (presumably #140608) made this pull request unmergeable. Please resolve the merge conflicts.

[Qelxiros]

@tgross35 This has been waiting for a review for two weeks. Should I reroll another reviewer? I'm not sure what the process is here, but I don't want it to slip through the cracks. Is this PR mergeable in it's current state or is there something more I need to implement? Obviously there are more functions in the ACP, but I want to make sure I'm on the right track with what I have.

[ChrisDenton]

Sorry for the delay, quite a lot of us were away for a week+. I can take an initial look at the Windows code.

[rustbot]

⚠️ Warning ⚠️

• This PR is based on an upstream commit that is 20 days old.

  It's recommended to update your branch according to the rustc-dev-guide.

[Qelxiros]

All good on the delay, and thanks for the review! Those issues should be good now. As a side note to whoever does the next review, there have been a lot of changes on the unix side too since the last review.