Refactor: boolean settings not treated as booleans

[jimklimov]

Some drivers (e.g. nutdrv_qx core or certain subdrivers) have options with values enabled/disabled that are magically parsed (with strchr('d') or similar) instead of just being booleans (with several standard string tokens per meaning) supported by NUT for a long time.

Revise the code for such cases and convert to common booleans where applicable. Perhaps add understanding of enabled/disabled as true/false to the common parsers, for backwards compatibility of existing configs.

Discovered during work on #2294.

See also #2811.