Stop using SALT_TYPE_AFS_LENGTH

Stop using SALT_TYPE_AFS_LENGTH

In krb5_init_creds_ctx and krb5_clpreauth_rock_st, use a boolean to
track whether we're still using the default salt instead of
overloading salt.length.  In preauth2.c, process afs3 salt values like
we would in krb5int_des_string_to_key, and set an s2kparams indicator
instead of overloading salt.length.  Also use an s2kparams indicator
in kdb_cpw.c's add_key_pwd.  Remove the s2k code to handle overloaded
salt lengths, except for a sanity check.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25837 dc483132-0cff-0310-8789-dd5450dbe970