Add lcpng_nl_sync.c that will house these functions. Their purpose is to
take state learned from netlink messages, and apply that state to VPP.
Some rearranging/plumbing was necessary to get logging to be visible in
this new source file.
Then, we add lcp_nl_neigh_add() and _del() which look up the LIP, convert
the lladdr and ip address from Netlink into VPP variants, and then add or
remove the ip4/ip6 neighbor adjacency.
