MDEV-20717 : Plugin system variables and activation options can break "mysqld --wsrep_recover"

Problem is that not all plugins are loaded when wsrep_recover is executed.
Thus, we allow unknown system variables and extra system variables during
wsrep_recover. Any unknown system variables would still be caught when
the server starts up normally after the SST.

sql/mysqld.cc

@@ -5417,6 +5417,10 @@ static int init_server_components()
       that there are unprocessed options.
     */
     my_getopt_skip_unknown= 0;
+#ifdef WITH_WSREP
+    if (wsrep_recovery)
+      my_getopt_skip_unknown= TRUE;
+#endif
 
     if ((ho_error= handle_options(&remaining_argc, &remaining_argv, no_opts,
                                   mysqld_get_one_option)))
@@ -5426,12 +5430,19 @@ static int init_server_components()
     remaining_argv--;
     my_getopt_skip_unknown= TRUE;
 
+#ifdef WITH_WSREP
+    if (!wsrep_recovery)
+    {
+#endif
       if (remaining_argc > 1)
       {
         fprintf(stderr, "%s: Too many arguments (first extra is '%s').\n",
                 my_progname, remaining_argv[1]);
         unireg_abort(1);
       }
+#ifdef WITH_WSREP
+    }
+#endif
   }
 
   if (opt_abort)