Shows a page for the user to type in their username and password.

• Caches the referring URL in $_SESSION['login_url'].
• Uses the Login template and language file with the login sub template.
• If you are using a wireless device, uses the protocol_login sub template in the Wireless template (protocol is 'wap2' or 'imode' since you can't login from regular wap).
• Accessed from ?action=login