std.fs: End iteration on Linux/WASI during Iterator.next when hitting `ENOENT`

`getdents` on Linux can return `ENOENT` if the directory referred to by the fd is deleted during iteration. Returning null when this happens makes sense because:

- `ENOENT` is specific to the Linux implementation of `getdents`
- On other platforms like FreeBSD, `getdents` returns `0` in this scenario, which is functionally equivalent to the `.NOENT => return null` handling on Linux
- In all the usage sites of `Iterator.next` throughout the standard library, translating `ENOENT` returned from `next` as null was the best way to handle it, so the use-case for handling the exact `ENOENT` scenario specifically may not exist to a relevant extent

Previously, ENOENT being returned would trigger `os.unexpectedErrno`.

Closes #12211

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