Simplify and fix kdcpreauth request_body callback

Simplify and fix kdcpreauth request_body callback

Alter the contract for the kdcpreauth request_body callback so that it
returns an alias to the encoded body instead of a fresh copy.  At the
beginning of AS request processing, save a copy of the encoded request
body, or the encoded inner request body for FAST requests.  Previously
the request_body callback would re-encode the request structure, which
in some cases has been modified by the AS request code.

No kdcpreauth modules currently use the request_body callback, but
PKINIT will need to start using it in order to handle FAST requests
correctly.

ticket: 7017
target_version: 1.10
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25473 dc483132-0cff-0310-8789-dd5450dbe970