Use `rustix` to replace all `libc` usage

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