the existing -q/--quiet option is renamed to --ignore-errors
* src/bin/ltlfilt.cc: Adjust option.
* src/bin/common_output.cc, src/bin/common_output.hh: Add a
quiet_output.
* bench/dtgbasat/prepare.sh: Rename -q to --ignore-errors..
* src/ltltest/remove_x.test: Use -q.
* NEWS: Mention this change.