obj: add header-offset field to object_type

this allows the object header to be placed anywhere within the larger
object structure.

the object system now also ensures that the object is zero-initialised
during allocation.

sched/task.c

@@ -11,6 +11,7 @@
 static struct object_type task_type = {
 	.ob_name = "task",
 	.ob_size = sizeof(struct task),
+	.ob_header_offset = offsetof(struct task, t_base),
 };
 
 static struct task *__kernel_task;