fixed warning when Warning: session_create_id(): Failed to create ID (#1291)

* fixed warning when Warning: session_create_id(): Failed to create new ID in /var/ssosp/lib/SimpleSAML/SessionHandlerPHP.php

* Use session_create_id() unconditionally

In master, we require PHP 7.2, which is guaranteed to provide `session_create_id()`. Therefore, we don't need the if clause any longer. Add also a warning when `session_create_id()` fails, and fall back gracefully to our old way to create session IDs.

Co-authored-by: Jaime Pérez Crespo <jaime.perez@uninett.no>