MEDIUM: move listener->frontend to bind_conf->frontend

Historically, all listeners have a pointer to the frontend. But since
the introduction of SSL, we now have an intermediary layer called
bind_conf corresponding to a "bind" line. It makes no sense to have
the frontend on each listener given that it's the same for all
listeners belonging to a same bind_conf. Also certain parts like
SSL can only operate on bind_conf and need the frontend.

This patch fixes this by moving the frontend pointer from the listener
to the bind_conf. The extra indirection is quite cheap given and the
places were this is used are very scarce.
diff --git a/src/cli.c b/src/cli.c
index 86fb2ea..4ccc49a 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -215,7 +215,7 @@
 			}
 		}
 
-		bind_conf = bind_conf_alloc(&global.stats_fe->conf.bind, file, line, args[2], &raw_sock);
+		bind_conf = bind_conf_alloc(global.stats_fe, file, line, args[2], &raw_sock);
 		bind_conf->level = ACCESS_LVL_OPER; /* default access level */
 
 		if (!str2listener(args[2], global.stats_fe, bind_conf, file, line, err)) {