Together with Mukund Sivaraman we found out PowerDNS sdig does not truncate

Together with Mukund Sivaraman we found out PowerDNS sdig does not truncate
trailing bits of EDNS Client Subnet mask.  So if you'd truncate something as
a /9, we'd have to use 2 bytes anyhow, but we would not zero the last 7 bits.

We do now. Thanks Mukund & ISC!

(cherry picked from commit d7da15c560946cadaadfc173b8964dd6b40932ed)