r219: Obtain new tickets if current ones are expired. Next part of fix for

r219: Obtain new tickets if current ones are expired.  Next part of fix for
bug 1208. Based on a fix from Guether Deschener.

Outstanding pieces:
- Heimdal FILE-based ccaches don't actually remove creds properly, so we
  need to code a check for this
- what if ticket expires between our check and when we use it?

Guenther has coded up fixes for these parts, but I still need
to review them, as I'm not totally comfortable with the solutions.
(This used to be commit ef008b9710e682f87f0bbf526d30eb5114264233)