FileObject instances: define meaning

As aftermath to VFS-523 we should really check what equals/hashcode implementation for all the different FileObject implementations needs to be used. Especially in the presence of solutions where FileName is not unique and FileSystemOptions might be different (but still point to the same target). Maybe some form of extended URL should be used to compare (or we define FileObject cannot be sanely compared). This also needs to factor in NullCache where each fileobject is a new instance.