resolved: rework how we handle truncation in the stub resolver

resolved: rework how we handle truncation in the stub resolver

When we a reply message gets longer than the client supports we need to
truncate the response and set the TC bit, and we already do that.
However, we are not supposed to send incomplete RRs in that case, but
instead truncate right at a record boundary. Do that.

This fixes the "Message parser reports malformed message packet."
warning the venerable "host" tool outputs when a very large response is
requested.

See: #6520