(is_member_in_recpt): Use is_member_in to simplify code.
(is_member_in_recpt_of): New static function.
(is_member_in_linked_recpt): New static function.
(interested_party): New static function. This also considers if
	the potential message recipient is a member of a recipient of a
	text that is linked to the text, via a misc-info or a aux-item.
(send_async_sub_recipient): Use interested_party to determine if
	the asynchronous message should be sent.
(send_async_deleted_text): Ditto.
(send_async_new_text_old): Ditto.
(send_async_new_text): Ditto.
(send_async_add_recipient): Ditto.
(text_read_access): Added const qualifiers.