MINOR: applet: add a new "owner" pointer in the appctx

This pointer indicates what stream-interface the appctx belongs to, just
like we have for the connections.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 39a4f9e..5462131 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -234,6 +234,7 @@
 {
 	si->ops = &si_embedded_ops;
 	si->end = &appctx->obj_type;
+	appctx->owner = si;
 }
 
 /* returns a pointer to the appctx being run in the SI or NULL if none */