MINOR: session: add a pointer to the session's origin

A session's origin is the entity that was responsible for creating
the session. It can be an applet or a connection for now.
diff --git a/src/stream.c b/src/stream.c
index 132e425..357e561 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -123,6 +123,7 @@
 
 	s->sess->listener = l;
 	s->sess->fe  = p;
+	s->sess->origin = &cli_conn->obj_type;
 
 	s->si[0].flags = SI_FL_NONE;
 	s->si[1].flags = SI_FL_ISBACK;