Integration of k5start/krenew functionality

[Greg Hudson]

After further consideration, it appears that kstart meets the current
needs for process credential management, as long as it is being
maintained, and there is no need to duplicate a subset of that
functionality in the krb5 tree.

This functionality might rear its head again in the context of a Unix
CCAPI port, for which there are other possible drivers, but that hasn't
been fleshed out enough for a full project proposal.

Thanks for the educational feedback.