Actually registers the user.

• Accessed at index.php?action=register2
• Performs various anti-spam checks to make sure it's a valid registration: registration is enabled, you didn't already register this session, you agree with the agreement (if it's enabled), etc.
• Validates all the input from the registration form
• Uses the Login language file
• Checks for additional information such as website, signature, etc. so the admin can require additional profile information at registration
• Displays the appropriate message to the user or logs them in, based on settings
• Sends appropriate email messages to the admin and the user (also based on settings)
• Logs the user in if the registration is set to immediate registration