More work on disk routines

I have made some more progress on the reworked disk routines.

I have almost completely refactored the code so that there are convenient routines for opening and reading a directory, retrieving each entry from a directory, finding a file in a directory, opening a file from its directory entry, as well as a convenience routine for finding, opening and loading a file all in one action.

I have also mostly implemented VFAT long file names.

Of course it doesn't actually work yet, but the infrastructure is all there now, and I will move into debugging mode when I get the chance.