We want to always resolve TGS requests even if the user mistakenly sends a
request for a service ticket where the fqdn part contain upper case letters.
The actual implementation follows hints set by KDC. When AP_REQ is done, KDC
sets KRB5_FLAG_ALIAS_OK and we obey it when looking for principals on TGS requests.
https://fedorahosted.org/freeipa/ticket/1577
