Fixes#420 reported by Salomon Sickert.
* bin/ltlcross.cc: Call determine_unknown_acceptance().
* spot/twaalgos/sccinfo.cc, spot/twaalgos/sccinfo.hh: Document
that one_accepting_scc()==-1 can mean "don't know", and update
determine_unknown_acceptance() to set one_acc_scc_.
* tests/core/ltlcross3.test: Add test case.
* NEWS: Mention the fixes.