MEDIUM: listeners: split the thread mask between receiver and bind_conf

With groups at some point we'll have to have distinct masks/groups in the
receiver and the bind_conf, because a single bind_conf might require to
instantiate multiple receivers (one per group).

Let's split the thread mask and group to have one for the bind_conf and
another one for the receiver while it remains easy to do. This will later
allow to use different storage for the bind_conf if needed (e.g. support
multiple groups).
diff --git a/src/proto_sockpair.c b/src/proto_sockpair.c
index 11cf353..7d4f7ee 100644
--- a/src/proto_sockpair.c
+++ b/src/proto_sockpair.c
@@ -157,7 +157,7 @@
 
 	rx->flags |= RX_F_BOUND;
 
-	fd_insert(rx->fd, rx->owner, rx->iocb, thread_mask(rx->settings->bind_thread) & all_threads_mask);
+	fd_insert(rx->fd, rx->owner, rx->iocb, thread_mask(rx->bind_thread) & all_threads_mask);
 	return err;
 
  bind_return: