fix handling of Rabin-like input for dnf_to_dca()
The bug is mentioned by Maximilien Colange in a comment to issue #317, but turned out to be unrelated to that original issue. * spot/twaalgos/totgba.cc (dnf_to_streett): Save the correspondence between the created states an the DNF clause in a named property. * doc/org/concepts.org, spot/twaalgos/totgba.hh: Mention the new property. * spot/twaalgos/cobuchi.cc (save_inf_nca_st): Rewrite using the named property. Relying on seen marks and trying to deduce the matching original clause could only work from plain Rabin. * tests/core/dca.test: Add the test from Maximilien. * NEWS: Mention the issue.