Merge pull request #4244 from rgacogne/dnsdist-check-cd

dnsdist: Add a `setCD` parameter to set cd=1 on health check queries

diff --cc pdns/README-dnsdist.md
Simple merge

diff --cc pdns/dnsdist-lua.cc
Simple merge

diff --cc pdns/dnsdist.cc
Simple merge

diff --cc pdns/dnsdist.hh
Simple merge