use Drupal query methods to access Drupal users table

By using CMS-specific queries to deal with CMS users in CMS-specific code, we may be able to remove the "user table name" setting, thus one less config option for users.

These patches cover this step for Drupal 6 and Drupal 7.

Reference this forum thread: http://forum.civicrm.org/index.php/topic,19773.0.html