I noticed that the command-line parsing framework has only OPT_BOOLEAN
and OPT_INTEGER, but no OPT_BOOLEAN_OR_INTEGER.
--no-<foo> works already with OPT_INTEGER. But for --<foo>, one
needs the
{ OPTION_INTEGER, short, long, &var, "n", description,
PARSE_OPT_OPTARG, NULL, 20 },
form.