fs: Implement RegInfo.FileName to return the on disk filename for a backend

Use it in make_backend_docs.py

bin/make_backend_docs.py

@@ -10,19 +10,12 @@ marker = "<!--- autogenerated options"
 start = marker + " start"
 stop = marker + " stop"
 
-# directory name to backend name
-dir_to_backend = {
-    "googlecloudstorage": "google cloud storage",
-    "amazonclouddrive": "amazon cloud drive",
-}
-
 def find_backends():
     """Return a list of all backends"""
     return [ x for x in os.listdir("backend") if x not in ("all",) ]
 
 def output_docs(backend, out):
     """Output documentation for backend options to out"""
-    backend = dir_to_backend.get(backend, backend)
     out.flush()
     subprocess.check_call(["rclone", "help", "backend", backend], stdout=out)