[krbdev.mit.edu #7190] Try harder to make keytab-based AS requests work

[Greg Hudson via RT]

This workaround may be a bit incomplete for the target audience: a realm 
which is migrating off DES, has stopped issuing DES keys for services, 
but hasn't updated the krbtgt principal to have anything other than a DES 
key.

If this is true, the realm may not have updated kadmin/changepw to 
contain additional enctypes either.  This ticket's change will make kinit 
-k work in such a scenario, but won't make k5srvutil change work.