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/proto_http.c b/src/proto_http.c
index 30468ee..7bf80ca 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -5035,7 +5035,7 @@
 	s->req.cons->err_type  = SI_ET_NONE;
 	s->req.cons->conn_retries = 0;  /* used for logging too */
 	s->req.cons->exp       = TICK_ETERNITY;
-	s->req.cons->flags    &= SI_FL_DONT_WAKE; /* we're in the context of process_session */
+	s->req.cons->flags    &= SI_FL_ISBACK | SI_FL_DONT_WAKE; /* we're in the context of process_session */
 	s->req.flags &= ~(CF_SHUTW|CF_SHUTW_NOW|CF_AUTO_CONNECT|CF_WRITE_ERROR|CF_STREAMER|CF_STREAMER_FAST|CF_NEVER_WAIT|CF_WAKE_CONNECT|CF_WROTE_DATA);
 	s->res.flags &= ~(CF_SHUTR|CF_SHUTR_NOW|CF_READ_ATTACHED|CF_READ_ERROR|CF_READ_NOEXP|CF_STREAMER|CF_STREAMER_FAST|CF_WRITE_PARTIAL|CF_NEVER_WAIT|CF_WROTE_DATA);
 	s->flags &= ~(SN_DIRECT|SN_ASSIGNED|SN_ADDR_SET|SN_BE_ASSIGNED|SN_FORCE_PRST|SN_IGNORE_PRST);