Login CCMD and menu checks/changes

This addresses most of the issues mentioned in https://zandronum.com/tracker/view.php?id=4355:

• The client can no longer make additional requests to login to the authentication server (i.e. the login CCMD) if they're already logged in, so as not to spam it repeatedly and potentially overload the authentication server.
• The login menu won't open if the client is already logged in and will inform them.
• If the client has a default username (i.e. login_default_user in Windows), the username in the login menu is automatically set to it when the menu is opened for the first time. Strictly speaking, if the menu_authusername CVar is still empty.