[dns-operations] TC=1 with RA=0 from a recursive resolver

[Florian Weimer]

We have received a bug report that our stub resolver does not retry over
TCP when asked to do so by some Google DNS resolvers:

  https://bugzilla.redhat.com/show_bug.cgi?id=1319296

The reason is that we check for RA=0 first and treat the server as
unusable if the bit is cleared.  Only after that, we check the TC bit.

We could easily check the TC bit first, but not setting RA=1 on a
recursive response when you are actually willing to provide recursion
looks like a server bug.

Thoughts?

Thanks,
Florian