MINOR: listener: move the interface to the struct settings

The interface is common to all listeners/receivers and is used to bind
the listening socket so it must be in the receiver settings and not in
the listener. This removes some unnecessary loops.
diff --git a/src/cli.c b/src/cli.c
index a455be1..d7ec79a 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1710,8 +1710,8 @@
 		if (fdtab[cur_fd].iocb == listener_accept) {
 			const struct listener *l = fdtab[cur_fd].owner;
 
-			if (l->interface) {
-				if_name = l->interface;
+			if (l->bind_conf->settings.interface) {
+				if_name = l->bind_conf->settings.interface;
 				if_nlen = strlen(if_name);
 			}