Do not error out if arg to -U option cannot be resolved, that is for

the plugin to decide.
There is no need for runas_user and runas_group to be global, make
them local to parse_args() instead.

src/sudo.c

@@ -94,7 +94,7 @@
 struct plugin_container policy_plugin;
 struct plugin_container_list io_plugins;
 struct user_details user_details;
-const char *list_user, *runas_user, *runas_group; /* extern for parse_args.c */
+const char *list_user; /* extern for parse_args.c */
 static struct command_details command_details;
 static int sudo_mode;