MINOR: listeners: introduce listener_set_state()

This function is used as a wrapper to set a listener's state everywhere.
We'll use it later to maintain some counters in a consistent state when
switching state so it's capital that all state changes go through it.
No functional change was made beyond calling the wrapper.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index 6cf35b7..bafe97d 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -118,7 +118,7 @@
 	}
 
 	/* the socket is now listening */
-	listener->state = LI_LISTEN;
+	listener_set_state(listener, LI_LISTEN);
 	return err;
 
  uxst_close_return:
@@ -142,7 +142,7 @@
 {
 	if (listener->state != LI_INIT)
 		return;
-	listener->state = LI_ASSIGNED;
+	listener_set_state(listener, LI_ASSIGNED);
 	listener->rx.proto = &proto_unix;
 	LIST_ADDQ(&proto_unix.listeners, &listener->rx.proto_list);
 	proto_unix.nb_listeners++;