Avoid looping when preauth can't be generated

Avoid looping when preauth can't be generated

If we receive a PREAUTH_REQUIRED error and fail to generate any real
preauthentication, error out immediately instead of continuing to
generate non-preauthenticated requests until we hit the loop count.

There is a lot of room to generate a more meaningful error about why
we failed to generate preauth (although in many cases the answer may
be too complicated to explain in an error message), but that requires
more radical restructuring of the preauth framework.

ticket: 6430
target_version: 1.10
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25469 dc483132-0cff-0310-8789-dd5450dbe970