Ensure the plugin works well with namespaces

I've made a few cosmetic adjustments:
- introduce debug, info, notice, warn and err loggers
- remove redundant logging, and set correct (conservative) log levels
- turn the sync-state error into a warning

And a little debt paydown:
- refactor sync-state into its own function, to be called instead of
  all the spot fixes elsewhere. It's going to be the case that
  sync-state is "the reconsiliation function".
- Fix a bug in lip->lip_namespace copy: vec_dup() there doesn't seem
  to work for me, use strdup() instead and make a mental note to
  reviist.

The plugin now works with 'lcpng default netns dataplane' in its
startup.conf; and with 'lcp default netns dataplane' as its first
command. A few of these fixes should go upstream, too, which I'll
do next.

lcpng_interface.h

@@ -26,11 +26,17 @@
 extern vlib_log_class_t lcp_itf_pair_logger;
 
 #define LCP_ITF_PAIR_DBG(...)                                                 \
-  vlib_log_notice (lcp_itf_pair_logger, __VA_ARGS__);
+  vlib_log_debug (lcp_itf_pair_logger, __VA_ARGS__);
 
 #define LCP_ITF_PAIR_INFO(...)                                                \
+  vlib_log_info (lcp_itf_pair_logger, __VA_ARGS__);
+
+#define LCP_ITF_PAIR_NOTICE(...)                                                \
   vlib_log_notice (lcp_itf_pair_logger, __VA_ARGS__);
 
+#define LCP_ITF_PAIR_WARN(...)                                                \
+  vlib_log_warn (lcp_itf_pair_logger, __VA_ARGS__);
+
 #define LCP_ITF_PAIR_ERR(...)                                                \
   vlib_log_err (lcp_itf_pair_logger, __VA_ARGS__);
 
@@ -187,6 +193,14 @@ void lcp_itf_ip6_add_del_interface_addr (ip6_main_t *im, uword opaque,
 					 u32 address_length,
 					 u32 if_address_index, u32 is_del);
 
+/* Sync all state from VPP to Linux device
+ *
+ * Note: in some circumstances, this syncer will (have to) make changes to
+ * the VPP interface, for example if its MTU is greater than its parent.
+ * See the function for rationale.
+ */
+void lcp_itf_pair_sync_state (lcp_itf_pair_t *lip);
+
 /*
  * fd.io coding-style-patch-verification: ON
  *