node_invoke_nodeapi recursion

I have had some weird problems, most of them involving hook_nodeapi. It is difficult to explain. For one of the problems see my comment at http://drupal.org/node/27007#comment-42637.

Another problem was that I broke the upload module, by calling node_load in hook_nodeapi($op = 'validate') in some module. This caused node_invoke_nodeapi to be called recursively, which doesn't work as expected because of the caching that module_implements does.

Sorry, I know all of this is kind of vague, but I can't explain it any better. You can take a look at my patch how I solved the problem. I think this patch does not have any impact on performance, and to prevent unexpected behavior involving hook_nodeapi this patch should be committed. If this patch is committed, a comment should be added to the code so that future developers will not change it back.