Because scc_info does not perform a full emptiness check, it is not
always able to tell whether an SCC is accepting if the acceptance
condition use Fin primitives.  This introduce is_rejecting_scc() in
addition to to is_accepting_scc().  Only one of them may be true, but
they can both be false if scc_info has no idea whether the SCC is
accepting.

* src/tgbaalgos/sccinfo.cc, src/tgbaalgos/sccinfo.hh: Implement
is_rejecting_scc().
* src/bin/ltlcross.cc, src/tgba/acc.cc, src/tgba/acc.hh,
src/tgbaalgos/dtgbacomp.cc, src/tgbaalgos/isweakscc.cc,
src/tgbaalgos/remfin.cc, src/tgbaalgos/safety.cc,
src/tgbaalgos/sccfilter.cc: Use it.
* src/tgbaalgos/dotty.cc: Use is_rejecting_scc() and is_accepting_scc()
to color SCCs.
* doc/org/oaut.org: Document the colors used.
* src/tgbatest/neverclaimread.test, src/tgbatest/readsave.test: Adjust
tests.
* src/tgbatest/sccdot.test: New test case.
* src/tgbatest/Makefile.am: Add it.