Use `rustix` to replace all `libc` usage

[Krysztal112233]

Recently I introduced rustix for refactoring and optimizing the code of pgrep because I found that the nix crate doesn't support the statx syscall, but rustix does.

After the related PR has been merged, I think maybe keeping just one of nix or rustix is sufficient, because they overlap significantly.

But I'm worried that replacing nix with rustix might cause some unexpected issues, after all, rustix is still very young.

I wonder if anyone has any related thoughts :)

cc @cakebaker @sylvestre @dezgeg @Bluemangoo @tertsdiepraam

[Krysztal112233]

And what attracts me most about rustix is that it provides Rust-style calls and focuses on type safety and IO safety.