SSO doesn't works on sub-directories

If trying to install under http://example.com/drupal or http://example.com/portal, SSO would return URLs like http://example.com/drupal/drupal/page/on/successful/login instead of http://example.com/drupal/page/on/successful/login (note the duplicated 'drupal/').

This patch fixes that and allows to install under root or under any directory.