hook_civicrm_config is not invoked when running background scripts

When executing backgrounds tasks, CiviCRM bootstraps before Drupal. This means that CRM_Core_Config::singleton() invokes hook_civicrm_config before Drupal is active – which means that hook_civicrm_config is necessarily a null-op.

This is relevant, for example, if you seek to register additional Smarty plugins for use in Smarty-based emails – if hook_civicrm_config doesn't run, then there's no sane and functional way to perform the registration.

This is similar to CRM-6948 – in both cases, the