Fix renewable ticket lifetimes

Fix renewable ticket lifetimes

Commit b0661f9176f5eb2644ba459e1b1e87d3dd502174 removed the starttime
hack in the EncTicketPart decoder.  Take this into account when
computing the old lifetime of a ticket we are renewing.  Without this
fix, we compute an old lifetime equal to the ticket end time, add that
to the current KDC time, and issue a ticket with a negative end time
due to wraparound.  Add a simple test to t_renew.py to detect this by
making sure that a renewed ticket is usable.

This bug appeared only on master and not as part of any release, so
there is no associated ticket.