BUG/MEDIUM: servers: Flag the stream_interface on handshake error.

When creating a new outgoing connection, if we're using ALPN and waiting
for the handshake completion to choose the mux, and for some reason the
handshake failed, add the SI_FL_ERR flag to the stream_interface, so that
process_streams() knows the connection failed, and can attempt to retry,
instead of just hanging.

This should be backported to 1.9.

src/backend.c

@@ -1089,6 +1089,8 @@ fail:
 	conn_stop_tracking(conn);
 	conn_full_close(conn);
 	conn_free(conn);
+	/* Let process_stream know it went wrong */
+	s->si[1].flags |= SI_FL_ERR;
 	return -1;
 }
 #endif