Project review: principals without keys

[Greg Hudson]

I have implemented kadmin support for making principal entries with no
keys, mainly so that we have a simple way to disable encrypted timestamp
and challenge for a principal.  The project page and candidate
implementation are at:

    http://k5wiki.kerberos.org/wiki/Projects/Principals_without_keys
    https://github.com/greghudson/krb5/commits/nokey

There are a few different design options, which were discussed on IRC
and are reflected in the project page.

Comments are appreciated.  Barring objections, I expect to push the
implementation to master later this week.