"On comments" checkboxes missing

The "On comments" checkboxes are not shown on node pages that do not allow comments. The reason for this is presumably that on the "Subscribe to this post" option the "On comments" checkbox is not needed. But on the other subscription options, for example for categories, the "On comments" checkbox is needed. The attached patch makes sure that the "On comments" checkboxes are always shown.

This does make the "Subscribe to this post" option a bit more ugly because it shows a "On comments" option even if there never will be any comments for that post. However that just adds slightly to the issue that already exists for that subscription option, see http://drupal.org/node/213457.