(handle_single_display_spec): Ensure the right value of

(handle_single_display_spec): Ensure the right value of
it->position is saved by push_it.
(pop_it): Restore it->object for GET_FROM_BUFFER and GET_FROM_STRING.
(reseat_1): Don't setup it->object twice.
(set_iterator_to_next): No need to set it->object after pop_it.
(move_it_to): Explicitly check to see if last move reached to_charpos.