MEDIUM: mux: Add an optional "reset" method.

Add a new method to mux, "reset", that is used to let the mux know the
connection attempt failed, and we're about to retry, so it just have to
reinit itself. Currently only the H1 mux needs it.
diff --git a/src/backend.c b/src/backend.c
index f494773..34a04c3 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1293,7 +1293,7 @@
 			return SF_ERR_INTERNAL;
 	}
 
-	if (!conn_xprt_ready(srv_conn)) {
+	if (!conn_xprt_ready(srv_conn) && !srv_conn->mux) {
 		/* the target was only on the stream, assign it to the SI now */
 		srv_conn->target = s->target;
 
@@ -1361,6 +1361,10 @@
 
 		assign_tproxy_address(s);
 	}
+	else if (!conn_xprt_ready(srv_conn)) {
+		if (srv_conn->mux->reset)
+			srv_conn->mux->reset(srv_conn);
+	}
 	else
 		s->flags |= SF_SRV_REUSED;