twaalgos: implement restrict_dead_end_edges_here()

Discussed in issue #587.

* spot/twaalgos/deadends.cc, spot/twaalgos/deadends.hh: New files.
* spot/twaalgos/Makefile.am, python/spot/impl.i: Add them.
* tests/core/deadends.test, tests/python/deadends.py: New files.
* tests/Makefile.am: Add them.
* spot/twa/acc.cc, spot/twa/acc.hh (keep_one_inf_per_branch): New
method.
* bin/autfilt.cc: Learn option --restrict-dead-end-edges.
* NEWS: Mention it.

tests/Makefile.am

@@ -270,6 +270,7 @@ TESTS_twa = \
   core/dupexp.test \
   core/exclusive-tgba.test \
   core/remprop.test \
+  core/deadends.test \
   core/degendet.test \
   core/degenid.test \
   core/degenlskip.test \
@@ -413,6 +414,7 @@ TESTS_python = \
   python/dbranch.py \
   python/declenv.py \
   python/decompose_scc.py \
+  python/deadends.py \
   python/det.py \
   python/dualize.py \
   python/ecfalse.py \