fuse/nodefs: mount root node.Node directly.

This obviates the nodefs.FileSystem type. To adjust for this API change,

  * call nodefs.MountRoot() on the Root() node of a to-be-mounted
    filesystem.

  * Move OnMount/OnUnmount methods to the type of the root node.

  * Stop embedding NewDefaultFileSystem() into file system
    objects. Usually FS objects no longer have to be exposed.