Fix never valid error condition in KDC

The TGS was incorrectly using authtime to compute renew_till for new
tickets, which was in turn leading to endtime potentially being equal to
starttime, which caused the TGS to return KRB5KDC_ERR_NEVER_VALID.

This happens when the TGT renewal lifetime is longer than the max renew
lifetime of any other services, after that much time (target services'
max renew life) passes.  The TGT is still good but TGS-REQs fail.