Fix bug #16870 with 'box' face in display strings.

Fix bug #16870 with 'box' face in display strings.

 src/xdisp.c (pop_it): Restore the it->face_box_p flag which could be
 reset by the face of the object just displayed.  See also bug#76.
 (get_next_display_element): If the string came from a display
 property, examine the box face attribute at it->position, not at
 it->current.pos, since the latter was not updated yet.
 (handle_face_prop): Improve commentary.