Support skip-plugin-load

- --disable-plugin-load or --skip-plugin-load will now reset plugin load
  list. Needed for future changes to mtr tests

sql/mysqld.cc

@@ -9316,6 +9316,8 @@ mysqld_get_one_option(int optid, const struct my_option *opt, char *argument)
     break;
   case OPT_PLUGIN_LOAD:
     free_list(opt_plugin_load_list_ptr);
+    if (argument == disabled_my_option)
+      break;                                    // Resets plugin list
     /* fall through */
   case OPT_PLUGIN_LOAD_ADD:
     opt_plugin_load_list_ptr->push_back(new i_string(argument));