Fix shutdown hooks not running

sig.c was calling plugin_run_hooks for the shutdown hooks, but the
shutdown hooks do not have registered callbacks in hook-scripts.c

Instead directly call plugin_script_run()

src/sig.c

@@ -330,7 +330,7 @@ void do_shutdown(shutop_t op)
 		;
 
 	/* All services and (non-critical) processes have stopped. */
-	plugin_run_hooks(HOOK_SVC_DN);
+	plugin_script_run(HOOK_SVC_DN);
 
 	if (in_cont) {
 		if (osheading)
@@ -380,7 +380,7 @@ void do_shutdown(shutop_t op)
 
 	/* Last chance to run scripts (all .so plugins have exited already) */
 	print(0, "Calling hook/svc/down scripts ...");
-	plugin_run_hooks(HOOK_SYS_DN);
+	plugin_script_run(HOOK_SYS_DN);
 
 	/* Reboot via watchdog or kernel, or shutdown? */
 	if (op == SHUT_REBOOT) {