Switch to the non-deprecated form of 'session_set_save_handler'.

As of PHP 8.4 the form with more than 2 arguments is deprecated.

This also does some initial work to make the functions behave closer to what SessionHandlerInterface describes.

* https://php.watch/versions/8.4/session_set_save_handler-alt-signature-deprecated
* https://wiki.php.net/rfc/deprecate_functions_with_overloaded_signatures
* https://www.php.net/manual/en/class.sessionhandlerinterface.php