For issue #570.
* spot/twaalgos/cleanacc.hh,
spot/twaalgos/cleanacc.cc (reduce_buchi_acceptance_set_here,
enlarge_buchi_acceptance_set_here): New functions.
* bin/autfilt.cc: Add options --reduce-acceptance-set and
--enlarge-acceptance-set.
* tests/core/basetred.test: New file.
* tests/Makefile.am: Add it.
* NEWS: Mention it.
