lint: Allow marking variables/arguments as unused
    
    Unused variables or arguments can indicate bugs, but they can also
    help document the code, in particular in case of signal handlers
    and destructuring.
    
    Account for this by keeping the error, but set up patterns that allow
    us to opt out of if for individual variables/arguments. For arguments
    we pick a '_' prefix, while for variables we go with a suffix instead,
    to not accidentally exempt private module-scope variables.