[getdns-api] invalid parameter return code

[Wiley, Glen]

I would like to add a return code "GETDNS_RETURN_INVALID_PARAMETER" for cases where something like a null pointer is passed for a callback function.  I think this is more helpful than returning a general error without getting so specific that we end up with too many return codes.

--
Glen Wiley
KK4SFV
Sr. Engineer
The Hive, Verisign, Inc.
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