Support for renameat2? #406

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.