Implement an fdatasync alternative on OS X

`fdatasync()` is not supported (as such) on OS X. An alternative using `fcntl()` is detailed in [a picoc issue](https://code.google.com/p/picoc/issues/detail?id=145).

We should provide the `fcntl()`-based alternative on OS X (and presumably iOS).

Start by locating the existing fdatasync implementation and its platform-specific conditionals in the unix package. Read the linked picoc issue for the fcntl()-based approach, then verify the change covers OS X and, if supported by the project, iOS; done means the alternative is provided on those platforms.

Written by the indexing model from the issue text.