HAMMER 26/many: More NFS support work, rename fixes

HAMMER 26/many: More NFS support work, rename fixes

* Add a fakename return argument to vop_nlookupdotdot().  This allows HAMMER
  to supply a fake name for historical (@@<timestamp>) transitions when
  regenerating the namecache topology.  The feature is needed when operating
  as a NFS server.

* Do not mark a vnode VROOT if it is a historical root rather than the
  realtime root.  This fixes confusion the namecache has when dealing
  with the root vnode of a HAMMER filesystem.

* Fix a bug in nrename().  ino_data.parent_obj_id was not being properly
  adjusted when moving directories.