Fix a few "unused parameter" warnings
Remove unused variables in src/filter.c:filter_apply_chain
Remove unused argument and variable from src/scanopt.c:scanopt_err
Remove unused variable from src/filter.c:filter_fix_linedirs
Remove a few `unused variable` warnings
Fix `implicit function declaration` warnings in tests
Fix `unused function` warnings in tests
Fix two "signed/unsigned" warnings
Fix `label unused` warning