Cracknames: use krb wrapper functions so it works with MIT

Cracknames: use krb wrapper functions so it works with MIT

Also avoid a silly game with directly modifying the principal and
then calling krb5_principal_unparse_flags to get out a string.
If we already assume it is a 2 components name and know what outcome we are
going to get, just go ahead and talloc_asprintf the linearized string.