RFE: `dirfd` / `*at` file operations support

[rektide]

Greetings!

I'd love to have dirfd / *at style file operations.

I have a system where I'm shuffling directories around some, and with dirfd, objects just need to know their parent & that's super simple. Right now I have to track a lot of extra state to properly recalculate PathBufs as the system changes.

[Berrysoft]

Unfortunately they are not cross-platform. We need to decide whether or how to make them possible on Windows.