MEDIUM: stream-int: add a flag indicating which side the SI is on

This new flag "SI_FL_ISBACK" is set only on the back SI and is cleared
on the front SI. That way it's possible only by looking at the SI to
know what side it is.
diff --git a/src/peers.c b/src/peers.c
index 48f5fc4..d16331d 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1163,6 +1163,9 @@
 	s->be = s->fe = p;
 	s->req.buf = s->res.buf = NULL;
 
+	s->si[0].flags = SI_FL_NONE;
+	s->si[1].flags = SI_FL_ISBACK;
+
 	si_reset(&s->si[0], t);
 	si_set_state(&s->si[0], SI_ST_EST);