k5start -K and ticket renewals

[Nico Williams]

Ideally the auto-renewal wake-up timer should be automatically set
from the TGT's lifetime (and libkrb5 should automatically handle any
faster expiration of non-initial tickets).  Then -K shouldn't be
needed.

The hard part is how to handle transient renewal errors, particularly
when the ticket's original lifetime was short (but renew lifetime
long).

Nico
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