Get rid of LimitedFs - FIXME needs docs on copying single files

If remote:path points to a file make NewFs return a sentinel error
fs.ErrorIsFile and an Fs which points to the parent.

Use this to remove the LimitedFs and just add this file to the
--files-from list.

This means that server side operations can be used also.

Fixes #518
Fixes #545

b2/b2.go

@@ -239,7 +239,8 @@ func NewFs(name, root string) (fs.Fs, error) {
 		}
 		obj := f.NewFsObject(remote)
 		if obj != nil {
-			return fs.NewLimited(f, obj), nil
+			// return an error with an fs which points to the parent
+			return f, fs.ErrorIsFile
 		}
 		f.root = oldRoot
 	}