VPP-19: Split the lookup.h IP_LOOKUP_NEXT enum.

VPP-19: Split the lookup.h IP_LOOKUP_NEXT enum.

IP4 and IP6 nodes currently shares the adj->lookup_next_index. That
has some issues, e.g. that one has to add non-functional nodes like
ip4-hop-by-hop and that anyone dynamically adding nodes to any of the
IP4/IP6 lookup nodes must ensure they add themselves to all relevant
nodes to ensure next index consistency.

This patch splits the IP_LOOKUP_NEXT into separate enums for IP4 and
IP6 with a common part for next-nodes used by both. It sets up other
IP nodes as siblings to avoid inconsistencies. This allows IP4 and IP6
lookup next nodes to evolve independently. The adj->lookup_next_index is
still shared, assuming that an IP4 adjacency isn't used by an
IP6 graph node.

Change-Id: I589b8364fe54e7a10c059b7ef9d6707eb0a345cc
Signed-off-by: Ole Troan <ot@cisco.com>