autfilt: better handling of chain of products with -B
Fixes #348, reported by Jeroen Meijer. * bin/autfilt.cc: If -B is used with many --product, degeneralize intermediate products as needed. * NEWS: Mention the change. * tests/core/prodchain.test: New file. * tests/Makefile.am: Add it. * spot/twa/acc.cc, spot/twa/acc.hh: Fix reporting of overflow. * tests/core/acc.cc: Adjust.