Reported by Simon Jantsch and David Müller.

* spot/twaalgos/isunamb.cc (is_unambiguous): Rewrite wihtout assuming
that the product of two accepting SCCs is accepting,  Also use
the result of is_accepting_scc()/is_rejectng_scc() when available.
* spot/twaalgos/sccinfo.cc, spot/twaalgos/sccinfo.hh: Make it
possible to check the acceptance of a unique SCC.
* tests/core/unambig.test: Add more test cases.