Set a default enctype for optimistic preauth

Set a default enctype for optimistic preauth

When the client application requests optimistic preauth for a preauth
type which uses the password, we don't have an etype-info2 to
interpret since we haven't talked to the KDC.  So we need to guess an
enctype, salt, and s2k parameters.  In 1.9 and prior, encrypted
timestamp contained code to use the first requested enctype in this
case, but encrypted challenge did not.  In 1.10 prior to this change,
neither mechanism uses a reasonable default.

Set a default enctype in krb5_init_creds_init so that all
password-based preauth mechanisms will use a reasonable default in the
optimistic preauth case.  The default salt and s2k parameters for this
case will be the principal-based default salt and the enctype-based
default parameters.

ticket: 7033
target_version: 1.10
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25504 dc483132-0cff-0310-8789-dd5450dbe970