do_sub_recpt.  Only send async-sub-recipient if the new
	argument want_async is true.
(do_sub_recpt): Code simplified by creation of perform_subtraction().
	New argument: want_async (see above).
(do_delete_text): Tell do_sub_recpt that we don't want any
	async-sub-recipient messages.
(sub_recipient): Tell do_sub_recpt to send async-sub-recipient messages.