dirfd: initial quick and dirty implementation for unix

[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]

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

error: an `#[unstable]` annotation here has no effect
    --> library/std/src/fs.rs:1375:1
     |
1375 | #[unstable(feature = "dirfd", issue = "120426")]
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information
     = note: `#[deny(ineffective_unstable_trait_impl)]` on by default

For more information about this error, try `rustc --explain E0412`.
[RUSTC-TIMING] std test:false 2.577
error: could not compile `std` (lib) due to 3 previous errors
Build completed unsuccessfully in 0:01:11