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_if_cli.c

@@ -111,6 +111,37 @@ VLIB_CLI_COMMAND(lcp_itf_pair_create_command, static) = {
     .function = lcp_itf_pair_create_command_fn,
 };
 
+static clib_error_t *
+lcp_lcp_sync_command_fn (vlib_main_t *vm, unformat_input_t *input,
+			 vlib_cli_command_t *cmd)
+{
+  unformat_input_t _line_input, *line_input = &_line_input;
+
+  if (!unformat_user (input, unformat_line_input, line_input))
+    return 0;
+
+  while (unformat_check_input (line_input) != UNFORMAT_END_OF_INPUT)
+    {
+      if (unformat (line_input, "on") || unformat (line_input, "enable"))
+	lcp_set_lcp_sync (1);
+      else if (unformat (line_input, "off") ||
+	       unformat (line_input, "disable"))
+	lcp_set_lcp_sync (0);
+      else
+	return clib_error_return (0, "unknown input `%U'",
+				  format_unformat_error, line_input);
+    }
+
+  unformat_free (line_input);
+  return 0;
+}
+
+VLIB_CLI_COMMAND (lcp_lcp_sync_command, static) = {
+  .path = "lcp lcp-sync",
+  .short_help = "lcp lcp-sync [on|enable|off|disable]",
+  .function = lcp_lcp_sync_command_fn,
+};
+
 static clib_error_t *
 lcp_lcp_auto_subint_command_fn (vlib_main_t *vm, unformat_input_t *input,
 				vlib_cli_command_t *cmd)