Default to using date_timezone module if available to handle DST properly; cleanup timezone code.

Most Drupal 6 sites have date_api and associated modules installed. One of these is date_timezone module which helpfully switches from Drupal 6's horrible second-offset model to timezone names — thus properly taking into account daylight savings changes.

This patch attempts to use this module if it is available so that CiviCRM doesn't get out of sync with PHP during daylight savings periods. It falls back to the old second-offset model if needed — but using code that handles 0-second offset properly and without several unused variables hanging around.

See PR: https://github.com/civicrm/civicrm-drupal/pull/365