MINOR: protocol: rename the ->listeners field to ->receivers

Since the listeners were split into receiver+listener, this field ought
to have been renamed because it's confusing. It really links receivers
and not listeners, as most of the time it's used via rx.proto_list!
The nb_listeners field was updated accordingly.
diff --git a/src/listener.c b/src/listener.c
index ff1d298..b24ee56 100644
--- a/src/listener.c
+++ b/src/listener.c
@@ -592,7 +592,7 @@
 	if (listener->state == LI_ASSIGNED) {
 		listener_set_state(listener, LI_INIT);
 		LIST_DEL(&listener->rx.proto_list);
-		listener->rx.proto->nb_listeners--;
+		listener->rx.proto->nb_receivers--;
 		_HA_ATOMIC_SUB(&jobs, 1);
 		_HA_ATOMIC_SUB(&listeners, 1);
 	}