Drupal 6 module port: Ubercart Restrict Qty

Today I ported the UC Restrict Qty Drupal module to Drupal 6. UC Restrict Qty is an Ubercart extension module which lets you add a very basic restriction to a product in your Ubercart store to limit the quantity of that product in the shopping cart to 1. This is useful for single purchase products or for instances where it just doesn't make sense for a customer to purchase more than one and you want to prevent it from happening.