Adding settings for URL prefix and random URI part

While using this module I wanted to (1) replace the default URL prefix protected by a free configurable string and (2) use a shorter random URI part because the UUID with 37 characters is too long for sending to customers etc.

May be my patch is useful for the public so I attached it here. It enhances url_access by the following features:

• Choose how to generate the random part of the URI: using the UUID() function of MySQL or the user_password() function of Drupal.
• Because the UUID() function is available from MySQL 4.1.2 on only the user_password() function will be used as a fallback if UUID() is not available.
• The URL path prefix for protected nodes which is by default protected is configurable.
• You may enter an additional message for the 403 page which unauthorized users will see when viewing the node without using the protected URL.