Support for renameat2?

On Linux, `cap-std` implements `Dir::rename` via `rustix::fs::renameat`, which is the `renameat` function.

Since Linux 3.15, there is another version of this call, `renameat2`, which supports an additional `flags` argument. `rustix` already supports this function as `rustix::fs::renameat_with`.

Could `cap-std` add support for this function as well?

## Additional Context

The external `cap-std-ext` crate implements atomic writes by writing a temporary file and renaming it in place. However, it is not possible to atomically create a file in this way.

`renameat2` supports the `RENAME_NOREPLACE` flag, which ensures that no existing file is overwritten by the rename. As such, an `atomic_create` could be implemented based on this change.

先从 cap-std 的 Dir::rename 和 rustix::fs::renameat_with 开始，然后比较文件系统操作在受支持平台上的公开方式。阅读 cap-std-ext 的原子写入实现，以了解 RENAME_NOREPLACE 的使用场景。在可用的平台上支持 renameat2 风格的标志，并且原子创建能够避免覆盖已有文件，即视为完成。

由索引模型根据 Issue 内容生成。