Move the MOD_SHUTDOWN event from shutdown_post_sync to shutdown_final,

so that entities that want to use the post_sync hook to write stuff
to devices and other tidy-up can do so before the device tree is
shot down.  eg: da doing a SYNC_CACHE etc.  This should get crashdumps
working on mpt devices again, and stops the ia64 boxes locking up
on regular shutdown when da tries to issue the scsi commands to mpt.

Obtained from:  njl, gibbs

sys/kern/kern_module.c

@@ -74,7 +74,7 @@ module_init(void *arg)
 
 	sx_init(&modules_sx, "module subsystem sx lock");
 	TAILQ_INIT(&modules);
-	EVENTHANDLER_REGISTER(shutdown_post_sync, module_shutdown, NULL,
+	EVENTHANDLER_REGISTER(shutdown_final, module_shutdown, NULL,
 	    SHUTDOWN_PRI_DEFAULT);
 }