Fix and simplify auth-indicator authdata module

Fix and simplify auth-indicator authdata module

Remove the authind_context count field.  The indicators list must be
null-terminated because it is freed with k5_free_data_ptr_list().
authind_internalize() didn't null-terminate the list, and the presence
of the count field made it appear that this wasn't a bug.  Use a
different scheme for setting *more in authind_get_attribute() to avoid
requiring an element count.

Also check more thoroughly for errors in authind_externalize() and
authind_internalize(), and remove some unnecessary pointer casts.

ticket: 8425