add bsn_flow_idle extension

Similar to standard OpenFlow idle timeouts with a flow-removed notification,
but does not remove the flow.
diff --git a/openflow_input/standard-1.3 b/openflow_input/standard-1.3
index 767594a..cbab686 100644
--- a/openflow_input/standard-1.3
+++ b/openflow_input/standard-1.3
@@ -250,6 +250,10 @@
     OFPFF_RESET_COUNTS = 0x4,
     OFPFF_NO_PKT_COUNTS = 0x8,
     OFPFF_NO_BYT_COUNTS = 0x10,
+
+    /* Non-standard, enabled by an experimenter message */
+    /* See the bsn_flow_idle input file */
+    OFPFF_BSN_SEND_IDLE = 0x80,
 };
 
 enum ofp_group(wire_type=uint32_t, complete=False) {