mkdir on nonexistent parent returns wrong error

[mkdir](https://pubs.opengroup.org/onlinepubs/9799919799/functions/mkdir.html) should return `ENOENT` if

> A component of the path prefix of *path* does not name an existing file or *path* is an empty string.

Currently, `mkdir("/a/b")` returns `EBADF` instead of `ENOENT` if `/a` does not exist.

This is required for [`std::fs::create_dir_all`](https://doc.rust-lang.org/stable/std/fs/fn.create_dir_all.html) to work.