Fix AS REQ start and end time to respect skew

[Greg Hudson]

On 05/22/2013 08:03 AM, Stef Walter wrote:
> The attached patch uses the unauthenticated time received during preauth
> to calculate these offsets.

Can you split this patch into two parts, instead of combining the
behavior changes with "In addition clean up some duplicated code"?  As
things stand, I can't easily tell what the new code is doing differently
from the old code.

Thanks.