New option: -c (Consider an unset change_name an error).

(unset_change_name_is_error): New variable.
(check_persons): Check change_name if unset_change_name_is_error is
set.  Set it if -r is given, or offer to set it if -i is given.
(main): Recognize -c and set unset_change_name_is_error if given.