Fix 1.6-* upgrade to 1.7.1

8e72e521d48ca04fd8db7d05318e2c55b9b2d278 (v1.7.1.2+) introduced a bug where osTicket version 1.6 would not send a cookie (by calling PHP session_start()) for the login page. Therefore, after unpacking the 1.7.1 source code, an upgrade would not be possible, because a login would never be processed correctly.