Implement content object policy functions

[elv-wayne]

  content_set_policy <object>               Set the policy on an existing
  <policy_path> [data]                      content object. This also sets the
                                            delegate on the object contract to
                                            itself.
  content_set_policy_delegate <object>      Set the policy delegate on the
  <delegate>                                object contract.
  content_get_policy <object>               Get the content object policy from
                                            the object metadata and the delegate
                                            from the object's contract meta
  content_clear_policy <object>             Remove content object policy from
                                            the object metadata and the delegate
                                            from the object's contract meta