refactor: I/O safety for sys/stat.rs

[SteveLauC]

What does this PR do

Add I/O safety to module sys/stat.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[thomaseizinger]

What is the intended migration for when I am currently calling fstatat with None? I am providing an absolute path as the argument and according to https://linux.die.net/man/2/fstatat, that means the first argument will be ignored. What should I be passing here now that I can no longer pass None?

[SteveLauC]

Hi @thomaseizinger

Before this PR, None will be translated to AT_FDCWD under this hood. After this PR, you can use https://docs.rs/nix/latest/nix/fcntl/constant.AT_FDCWD.html.

I am providing an absolute path as the argument and according to https://linux.die.net/man/2/fstatat, that means the first argument will be ignored.

Since the kernel will ignore this argument, it doesn't matter what it is, so technically, you can use anything.

But, AT_FDCWD is what you want if you want do a 1:1 migration.