MINOR: listener: prefer to retrieve the socket's settings via the receiver

Some socket settings used to be retrieved via the listener and the
bind_conf. Now instead we use the receiver and its settings whenever
appropriate. This will simplify the removal of the dependency on the
listener.
diff --git a/src/proto_sockpair.c b/src/proto_sockpair.c
index 4030c33..90300cc 100644
--- a/src/proto_sockpair.c
+++ b/src/proto_sockpair.c
@@ -126,7 +126,7 @@
 	listener->state = LI_LISTEN;
 
 	fd_insert(fd, listener, listener->rx.proto->accept,
-	          thread_mask(listener->bind_conf->settings.bind_thread) & all_threads_mask);
+	          thread_mask(listener->rx.settings->bind_thread) & all_threads_mask);
 
 	return err;