Protect VPP -> Linux state propagation behind flag

Introduce lcp_main.lcp_sync, which determines if state changes made
to interfaces in VPP do or don't propagate into Linux.

- Add a startup.conf directive 'lcp-sync' to enable at startup time.
- Add CLI.short_help = "lcp lcp-sync [on|enable|off|disable]",
- Show the current value in "show lcp".

Gate changes in mtu, state and address on lcp_lcp_sync().

When the operator issues 'lcp lcp-sync on', it is prudent to do a
one-off sync of all interface attributes from VPP into Linux.
For this, add a lcp_itf_pair_sync_state_all() function.

lcpng.c

@@ -69,6 +69,26 @@ int lcp_set_default_ns(u8 *ns) {
     return 0;
 }
 
+void
+lcp_set_lcp_sync (u8 is_auto)
+{
+  lcp_main_t *lcpm = &lcp_main;
+
+  lcpm->lcp_sync = (is_auto != 0);
+
+  // If we set to 'on', do a one-off sync of LCP interfaces
+  if (is_auto)
+    lcp_itf_pair_sync_state_all ();
+}
+
+int
+lcp_lcp_sync (void)
+{
+  lcp_main_t *lcpm = &lcp_main;
+
+  return lcpm->lcp_sync;
+}
+
 void
 lcp_set_lcp_auto_subint (u8 is_auto)
 {