Allow sites to use a variable other than the drupal_get_private_key variable

The Drupal private key feature is great for people to get up and running quickly but it's not great for security because it is often stored in the database. You can theoretically remove it from the db and incldue it in settings.php's $conf variable, but that feels risky to me since it might sneak into the database or might get leaked by another module.

The attached plugin (renamed with .txt) uses a different variable specifically for encryption.

Totally untested (needs automated tests as well, probably).
Needs a hook_requirements integration.