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_sockpair.c b/src/proto_sockpair.c
index fc26a80..eb7ef86 100644
--- a/src/proto_sockpair.c
+++ b/src/proto_sockpair.c
@@ -89,7 +89,7 @@
 {
 	if (listener->state != LI_INIT)
 		return;
-	listener->state = LI_ASSIGNED;
+	listener_set_state(listener, LI_ASSIGNED);
 	listener->rx.proto = &proto_sockpair;
 	LIST_ADDQ(&proto_sockpair.listeners, &listener->rx.proto_list);
 	proto_sockpair.nb_listeners++;
@@ -175,7 +175,7 @@
 		goto err_return;
 	}
 
-	listener->state = LI_LISTEN;
+	listener_set_state(listener, LI_LISTEN);
 	return err;
 
  err_return: