ARP: add feature arc

- arp-input, registered with the ethernet protocol dispatcher, performs
basic checks and starts the arc
- arp-reply; first feature on the arc replies to requests and learns
from responses (no functional change)
- arp-proxy; checks against the proxy DB

arp-reply and arp-proxy are enabled when the interface is appropriately
configured.

Change-Id: I7d1bbabdb8c8b8187cac75e663daa4a5a7ce382a
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/ip/ip4_forward.c b/src/vnet/ip/ip4_forward.c
index d8eed67..9c5524f 100644
--- a/src/vnet/ip/ip4_forward.c
+++ b/src/vnet/ip/ip4_forward.c
@@ -515,6 +515,12 @@
 
   vnet_feature_enable_disable ("ip4-multicast", "ip4-not-enabled",
 			       sw_if_index, !is_enable, 0, 0);
+
+  {
+    ip4_enable_disable_interface_callback_t *cb;
+    vec_foreach (cb, im->enable_disable_interface_callbacks)
+      cb->function (im, cb->function_opaque, sw_if_index, is_enable);
+  }
 }
 
 static clib_error_t *