Add skeleton of Linux CP Netlink Listener

Register lcp_nl_init() which adds interface pair add/del callbacks.

lcb_nl_pair_add_cb: Initiate netlink listener for first interface in its
  netns. If subsequent adds are in other netns, issue a warning. Keep
  refcount.
lcb_nl_pair_del_cb: Remove listener when the last interface pair is
  removed.

Socket is opened, file is added to VPP's epoll, with lcp_nl_read_cb()
and lcp_nl_error_cb() callbacks installed.
- lcp_nl_read_cb() calls lcp_nl_callback() which pushes netlink messages
  onto a queue and issues NL_EVENT_READ event, any socket read error
  issues NL_EVENT_READ_ERR event.
- lcp_nl_error_cb() simply issues NL_EVENT_READ_ERR event.

Then, initialize a process node called lcp_nl_process(), which handles:
- NL_EVENT_READ and call lcp_nl_process_msgs()
  - if messages are left in the queue, reschedule consumption after M
    msecs. This allows new netlink messages to continuously be read from
    the kernel, even if we have lots of messages to consume.
- NL_EVENT_READ_ERR and close/reopens the netlink socket.

lcp_nl_process_msgs() processes up to N messages and/or for up to M
msecs, whichever comes first. For each, calling lcp_nl_dispatch().

lcp_nl_dispatch() ultimately just throws the message away after
logging it with format_nl_object()

CMakeLists.txt

@@ -53,6 +53,14 @@ add_vpp_plugin(lcpng_if
   lcpng
 )
 
+add_vpp_plugin(lcpng_nl
+  SOURCES
+  lcpng_netlink.c
+
+  LINK_LIBRARIES
+  lcpng
+)
+
 add_vpp_plugin(lcpng_unittest
   SOURCES
   test/lcpng_unittest.c