Improve protected node

Hello,

This task is intended to store the ideas of improvement of the module. When you see anything you think it will improve the module (documentation needed, coding style, refactoring, ...) post a comment or edit the issue body.

• Using the Drupal API to avoid database requests using MySQL
• The functions protected_node_enter_any_password (protected_node.fork.inc) and protected_node_enterpassword (protected_node.redirect.inc) have a lot of code in common (which also should be cleaned)

Thanks.