DVR: run L3 output features

- rename l2_bridged to is_dvr. Including on the ip.api
    this was new in the 18.01 release so no compatability issues.
- steal the free space in vnet_buffer_opaque_t for use with flags.
- run the ipX-output feature arc from the DVR DPO

Change-Id: I040e5976d1dbe076fcdda3a40a7804f56337ce3f
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/ip/ip6_forward.c b/src/vnet/ip/ip6_forward.c
index 29cd3ca..a7e7930 100644
--- a/src/vnet/ip/ip6_forward.c
+++ b/src/vnet/ip/ip6_forward.c
@@ -666,7 +666,7 @@
 VNET_FEATURE_ARC_INIT (ip6_output, static) =
 {
   .arc_name  = "ip6-output",
-  .start_nodes = VNET_FEATURES ("ip6-rewrite", "ip6-midchain"),
+  .start_nodes = VNET_FEATURES ("ip6-rewrite", "ip6-midchain", "ip6-dvr-dpo"),
   .arc_index_ptr = &ip6_main.lookup_main.output_feature_arc_index,
 };