These changes implemnt FAT file shrinkage as needed to fully support ftruncate().

Squashed commit of the following:

    fs/fat:  Resolves issues with truncating the cluster chain when shrinking files via ftruncate().
    fs/fat:  First cut at implementation file shrinkage logic needed to support ftruncate().  Certainly shrinks the file size but it does not appear to correctly disconnect the cluster chains.
    fs/fat:  Restructure some functions in files to better support forthcoming file shrinkage logic.  Put framework for file shrinkage in place.  That logic is incomplete on initial commit.