Attempt to improve Unix filesystem support

Add some new methods to PK3StructuredReader and DirectoryReader, which get the filename with the correct case, and get a file at a particular path.
Replace backslashes in modeldef model paths with forward slashes, instead of doing the opposite, which was preventing some models from loading.

I don't know whether or not this is a good solution or not, since I don't know the UDB codebase very well.