Required for other representations of FileSystems, ie LocalFileSystem
Introduces FileSystemHandle for the same behaviour as FileHandle and
DirHandle.
Requires the following to hook into file/dir lookup:
```
int open(FileHandle **file, const char *filename, int flags)
int open(DirHandle **dir, const char *path)
```
This hook is provided by the FileSystem class, so requires no changes
from implementations.
Christopher Haster