BUG/MEDIUM: proxy: properly stop backends on soft-stop

BUG/MEDIUM: proxy: properly stop backends on soft-stop

On soft-stop, we must properlu stop backends and not only proxies with at
least a listener. This is mandatory in order to stop the health checks. A
previous fix was provided to do so (ba29687bc1 "BUG/MEDIUM: proxy: properly
stop backends"). However, only stop_proxy() function was fixed. When HAproxy
is stopped, this function is no longer used. So the same kind of fix must be
done on do_soft_stop_now().

This patch partially fixes the issue #1874. It must be backported as far as
2.4.