Verify that the full @getdnsapi/getdns API has been implemented

[joelpurra]

While writing tests, a few "magic numbers" popped up. Turns out no all @getdnsapi/getdns API constants have been implemented in getdns-node/src/GNConstants.cpp.

• Might more be missing?
• How can changes in the @getdnsapi/getdns API -- past or future -- be detected on the code level, so that the "diff" can be implemented in getdns-node?

The closest thing that comes to mind is to diff getdns/src/getdns/getdns.h.in between releases. What the changes actually mean would then be lookup up in the API documentation. Is that enough?