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.

NewChild also adds inode as a child.

Code that did:

  ch := n.Inode().New(isDir, fsNode)
  n.Inode().AddChild(name, ch)

should be changed to look like

  ch := n.Inode().NewChild(name, isDir, fsNode)

If we wait longer, we will probably wait indefinitely.

Versions have to integrate with the build/version control system,
which the go tool does not provide.

This could happen to unionfs with ClientInodes set, and manipulations
that skipped the backing store.  Added a test.