Port to drupal 6.0

I ran the coder (code review) module and it led me through a few changes.

There are some remaining issues:

signature_forum.module

      severity: critical Line 359: new hook_theme() function is required to register theme_ functions (Drupal Docs)
      function theme_signature_forum($signature) {

      severity: critical Line 38: The menu system has been completely over-hauled in 6.x. (Drupal Docs)
        if ($may_cache) {

      severity: critical Line 39: The menu system has been completely over-hauled in 6.x. (Drupal Docs)
          $items[] = array(

signature_forum.install

      severity: critical click to read more Line 8: A new schema API has been added in 6.x (Drupal Docs)
        switch ($GLOBALS['db_type']) {

      Explanation: This patch caused changes to the format of hook_install(), hook_uninstall(), and hook_update_N(). No longer are switch statements done on $GLOBALS['db_type']; instead, use the variety of schema API functions to perform table manipulation.

      severity: critical click to read more Line 29: A new schema API has been added in 6.x (Drupal Docs)
        switch ($GLOBALS['db_type']) {