Simplify the settings for filesystem paths

• Hosting environments like WPEngine don't permit (extra) .php files located in the webroot (apparently)
• Caching generated content there means potential for info leakage or execution via input variables
• Filesystem caching can be inefficient compared to other caches
• Herb has done work to get Smarty caching via Redis?
• https://www.drupal.org/node/2570335
• https://forum.civicrm.org/index.php?topic=35125.0;nowap
• https://issues.civicrm.org/jira/browse/CRM-17401
• https://github.com/freeform/civicrm-drupal-pantheon/blob/4.6.x/patches/smarty-redis-civi-cache.patch
• would Smarty 2 choke if we just set the path to redis:// ?
• .php extension is tacked on here, may be other places to change this