fuse/nodefs: clarify types.
fuse/nodefs: namespace functions now return *Inode.

This makes it impossible to forget to create an Inode to match the
FsNode.

This is an API change. To update, update function signatures, and change
  
  me.Inode().NewChild("name", isdir, child)
  return child, fuse.OK

to

  return me.Inode().NewChild("name", isdir, child), fuse.OK

in Mkdir, Mknod, Create, Symlink and Link methods.