disable page cache through the API rather than hacking global conf

Same idea as https://www.drupal.org/node/2465073 for the Captcha module, when Honeypot needs to disable the page cache for the current page, it would be better to call:

drupal_page_is_cacheable(FALSE);

...rather than the way it's currently done by modifying the $conf global:

$GLOBALS['conf']['cache'] = 0;

Apart from anything else, this makes it much easier to work out what's mysteriously disabling page cache on a site.

Patch on the way.