Our first Netlink syncer!

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.

CMakeLists.txt

@@ -56,6 +56,7 @@ add_vpp_plugin(lcpng_if
 add_vpp_plugin(lcpng_nl
   SOURCES
   lcpng_netlink.c
+  lcpng_nl_sync.c
 
   LINK_LIBRARIES
   lcpng