FoldbackStrip::update_send_box() GUI relies on ->fed_by() to
determine if a FB bus has a send from a route.
Under the hood this uses direct_feeds_according_to_reality()
and InternalSend::feeds(). When `allow_feedback` is set a send is
assumed to not directly feed a target (to allow loopbacks).
This mode must not be used for foldback sends.