Make CiviCRM compatible with Drupal cache

This patch makes CiviCRM compatible with the Drupal cache. It has not been tested exhaustively, but seems to work with both user registration and anonymous event registration with the drupal cache enabled.

For civicrm/* pages, the drupal cache is disabled. For the user/register page, CiviCRM's qfKey is disabled, as this feature is not compatible with drupal user registration (presumably due to the session being reset).