vlc_fs: rework the VLC readdir API so it doesn't use dirent on Windows

dirent is not a standard header on Windows. In mingw-w64 it also depends if the CRT is compiled with UNICODE or not.

We can use the native API that is available even in UWP/UAP builds.

A vlc_DIR type is introduced so DIR is not used without dirent API and we can have our own internal representation. This also avoids some dirty casts between DIR and the old win32 structure.

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v4:

• exported vlc_closedir() and vlc_rewinddir() for consistency
• make the win32 vlc_DIR private
• fixes from previous patch orders