[krbdev.mit.edu #8539] Preauth tryagain should copy KDC cookie

[Greg Hudson via RT]

RFC 6113 requires that "The client MUST copy the exact cookie 
encapsulated in a PA-FX-COOKIE data element into the next message of the 
same conversation."  When we try again after a mechanism-specific error 
(which in practice means a PKINIT error), we do not copy the KDC cookie.  
We should fix this for better performance, but we do not need to 
backport the fix as PKINIT does not require the use of cookies.