These entries returned by `readdir` are legacy of Unix API, we don't
really need them. Most APIs do not return them (for example Java
`Files.newDirectoryStream` or Python `os.listdir`).
* add `nextDirEntry` which returns `Maybe String`, so `Nothing` on
the end of directory unlike `dirEntry` which returns unspecified error
on the end of directory
* `dirEntry` is deprecated now, but not removed because compiler depends on it
* native implementation of `dirEntry` is patched to explicitly reset `errno`
before the `readdir` call: without it end of directory and error were
indistinguishable
* test added
