make the dns resolver configurable for v4 or v6 (or both), update docs

made the dns detector less special: it can now get configured as any other detector.

 this is most useful to give either family:INET or family:INET6 options, so dyndnsc will behave less confused as before (when it regularly updated IPs, just because of comparing 2 different kinds of IPs). there is also a hostname:host.name.to.query.com option, but it defaults to hostname from first updater, so one usually does not need to give it.

 refactored some code into a separate parse_detector_opts function to avoid duplication.