kernel: add support for recursive object destruction (without recursion)

this system makes it possible for an object that forms part of a tree
to be safely recursively destroyed without using recursion.

include/kernel/object.h

@@ -52,7 +52,10 @@ enum object_type_flags {
 };
 
 struct object_ops {
-	kern_status_t (*destroy)(struct object *obj);
+	kern_status_t (*destroy)(struct object *obj, struct queue *q);
+	kern_status_t (*destroy_recurse)(
+		struct queue_entry *entry,
+		struct object **out);
 };
 
 struct object_type {