these two patches add support for FAST to the Kerberos provider as
requested in trac ticket #237. The first patch just modifies some helper
functions in krb5_child so that they can be used by the second patch.
We support FAST only with MIT Kerberos 1.8 or later, because with 1.7 it
is not possilbe to implement the 'try' option anthentication would fail
if the server does not support FAST.