MEDIUM: channel: add a new flag "CF_ISRESP" for the response channel

This flag designates the response channel. This will be used to know
what channel we're seeing and finding our way back to the session.
diff --git a/src/session.c b/src/session.c
index a924e8c..4825cb3 100644
--- a/src/session.c
+++ b/src/session.c
@@ -502,6 +502,8 @@
 	s->req.analyse_exp = TICK_ETERNITY;
 
 	channel_init(&s->res);
+	s->res.flags |= CF_ISRESP;
+
 	s->res.prod = &s->si[1];
 	s->res.cons = &s->si[0];
 	s->res.analysers = 0;