Further refinement for Drupal 6

This patch takes the current HEAD version, which is Drupal-6-compatible as of the commit on July 5, and makes the following changes:

• properly closes tags in the help hook
• fixes indentation in paging_form_alter()
• standardizes variable naming in _paging_nodeapi (e.g., $node_body instead of $nodebody)
• adds some additional comments
• changes the tests from $node->in_preview (which does not exist in Drupal 6) to $node->build_mode == NODE_BUILD_PREVIEW
• updates paging_link() to have a single if statement and to comply with Drupal 6
• changes the test in paging_link_alter from $node->teaser to !empty($node->teaser) for performance
• fixes link in filter description so instead of pointing to nonexistent admin/settings/paging it points to the content type setting page, since that is where the page break variable is set now
• removes unused d() debugging function