Sync IPv4 and IPv6 addresses from VPP to LCP

There are three ways in which IP addresses will want to be copied
from VPP into the companion Linux devices:

1) set interface ip address ... adds an IPv4 or IPv6 address
  - this is handled by lcp_itf_ip[46]_add_del_interface_addr() which
    is a callback installed in lcp_itf_pair_init()
2) set interface ip address del ... removes them
  - also handled by lcp_itf_ip[46]_add_del_interface_addr() but
    curiously there is no upstream vnet_netlink_del_ip[46]_addr() so
    I wrote them inline here - I will try to get them upstreamed, as
    they appear to be obvious companions in vnet/device/netlink.h
3) Upon LIP creation, it could be that there are L3 addresses already
   on the VPP interface. If so, set them with lcp_itf_set_interface_addr()

This means that now, at any time a new LIP is created, its state from
VPP is fully copied over (MTU, Link state, IPv4/IPv6 addresses)!

At runtime, new addresses can be set/removed as well.

CMakeLists.txt

@@ -28,6 +28,7 @@ add_vpp_library(lcpng
   SOURCES
   lcpng_interface.c
   lcpng_adj.c
+  lcpng_if_sync.c
   lcpng.c
 
   LINK_LIBRARIES
@@ -44,7 +45,6 @@ add_vpp_plugin(lcpng_if
   lcpng_if_api.c
   lcpng_if_cli.c
   lcpng_if_node.c
-  lcpng_if_sync.c
 
   API_FILES
   lcpng_if.api