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.
diff --git a/src/proxy.c b/src/proxy.c
index 93c7620..60f9231 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -2198,6 +2198,7 @@
 /* perform the soft-stop right now (i.e. unbind listeners) */
 static void do_soft_stop_now()
 {
+	struct proxy *p;
 	struct task *task;
 
 	/* disable busy polling to avoid cpu eating for the new process */
@@ -2227,6 +2228,15 @@
 
 	thread_release();
 
+	/* Loop on proxies to stop backends */
+	p = proxies_list;
+	while (p) {
+		HA_RWLOCK_WRLOCK(PROXY_LOCK, &p->lock);
+		proxy_cond_disable(p);
+		HA_RWLOCK_WRUNLOCK(PROXY_LOCK, &p->lock);
+		p = p->next;
+	}
+
 	/* signal zero is used to broadcast the "stopping" event */
 	signal_handler(0);
 }