[getdns-api] Add a GETDNS_RETURN_NOT_IMPLEMENTED return code

[Willem Toorop]

Dear spec developers,

Our implementation (at getdnsapi.net) now offers the possibility to
compile without recursing resolution modus; for applications that are
interested in stub resolution only.  When recursing resolution mode is
tried to be set, our library currently returns a
GETDNS_RETURN_GENERIC_ERROR (by lack of something better).  I would
rather return something more specific that indicates unavailability of
the requested configuration.

The GETDNS_RETURN_NOT_IMPLEMENTED return code would fit this situation
much better and could also be used with currently unimplemented features
(such as TSIG support and call_debugging etc).

How do you feel about this?

-- Willem