Related to issue #587.
* spot/twaalgos/postproc.cc, spot/twaalgos/postproc.hh: Add
support for option "rde".
* bin/spot-x.cc, NEWS: Mention it.
* tests/core/deadends.test, tests/core/ltl2tgba2.test,
tests/python/atva16-fig2a.ipynb, tests/python/deadends.py: Adjust test
cases to reflect the improvement.
* tests/core/ltlsynt.test: Also adjust this test case, which is the
only one worsened. Some extra gates are generated when translating
GFa<->GFb with --algo=ds or --algo=sd. Issue #588 would be one way to
fix that.