fixes around fat32 and lfn support

[mvo5]

This is a drive-by PR.

I was investigating a corrupted vfat based system today and needed something to poke programmatically at the broken image. Your library was exactly the right tool (modulo some small issues that I had to tweak, see below), many thanks for it!

The image was fat32 so I added (hacky) support to get the root directory for fat32. I also fixed some issues in the lfn (long-file-name) reading. For my purposes this works well enough and I just wanted to share my fixes as they may be useful to other.

Cheers, Michael