Francis pointed out in RT #34313 and #32406 that the code for setting the CD bit
in resolver.c doesn't match the comment, or the intention.

Currently it always sets CD=1 on queries to forwarders.  By default, it
should start with CD=0 and retry with CD=1 if it gets a SERVFAIL. 

We should probably add another fetch option to make it *always* use CD=1, and
continue to use NOCDFLAG when we want it only to use CD=0 and then give up.
Possibly these options should be configurable in a "server" statement.