Use drupal access checks instead of disabling edit form

This patch does two things when the time limit for editing a comment has passed:

1. Uses a drupal access check and returns access denied instead of making the edit form read-only.
2. Removes the edit links from the comment so the user can't even try to edit the comment. This uses hook_link_alter, which required a patch to drupal in 6.17 to make it work. See #374463: Alter comment links..

It should be no problem on drupal < 6.17 - it will still work but the edit links will not be removed (like the module now). But when they are clicked the user will get an access denied message instead of a disabled edit form.

Users on drupal >= 6.17 will not see edit links for comments they cannot edit, so they will only ever see the access denied message if they hand craft the edit URL.