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 (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.
Merge pull request #26 from nsupdate-info/dns-af-supp

make the dns resolver configurable for v4 or v6 (or both), update docs (try 2)