User who has permission to edit node type, but not 'create blocks for nodes' will overwrite block settings with defaults

If a user has the permission to edit the content type, but not fiddle with the block settings, things like visibility are returned to their default values. The attached patch, which will have incorrect line numbers as it is patched against the D6 port (#254339), adds a permission check to the hook_nodeapi update/insert op to prevent this.