Avoid calling scc_info::determine_unknown_acceptance on the product, as
suggested in #188.

* spot/twaalgos/isunamb.cc (is_unambiguous): Rewrite.
* tests/core/unambig.test: Add the automaton from #188.
* NEWS: Mention the improved function.
* spot/twaalgos/mask.cc,
spot/twaalgos/mask.hh (mask_keep_accessible_states): New function.