Integration with Authcache module

From the Authcache issue queue: #566086: Integration with the cache module.

Similar to integrating with CacheRouter, the easiest step would be to wrap the page_cache_fastpath() function in cache.inc with:

if (!function_exists('page_cache_fastpath')) {
  function page_cache_fastpath() {
    // ...
  }
}

This way Authcache can include cache.inc without any conflicts and users do not have to hack the file.