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/sock.c b/src/sock.c
index 1614d95..5899d44 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -385,8 +385,8 @@
 			options |= SOCK_XFER_OPT_V6ONLY;
 	}
 
-	if (l->interface)
-		if_namelen = strlen(l->interface);
+	if (l->bind_conf->settings.interface)
+		if_namelen = strlen(l->bind_conf->settings.interface);
 #ifdef USE_NS
 	if (l->netns)
 		ns_namelen = l->netns->name_len;
@@ -396,7 +396,7 @@
 		if ((options == xfer_sock->options) &&
 		    (if_namelen == xfer_sock->if_namelen) &&
 		    (ns_namelen == xfer_sock->ns_namelen) &&
-		    (!if_namelen || strcmp(l->interface, xfer_sock->iface) == 0) &&
+		    (!if_namelen || strcmp(l->bind_conf->settings.interface, xfer_sock->iface) == 0) &&
 #ifdef USE_NS
 		    (!ns_namelen || strcmp(l->netns->node.key, xfer_sock->namespace) == 0) &&
 #endif