Fix 1.6-* upgrade to 1.7.1

8e72e521 (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.