Extract code preparing a delegation response to a separate function

Extract code preparing a delegation response to a separate function

Changes introduced by the previous two commits make the parts of
query_delegation() and query_zone_delegation() which prepare a
delegation response functionally equivalent.  Extract this code into a
separate function, query_prepare_delegation_response(), and then call
the latter from both query_delegation() and query_zone_delegation() in
order to reduce code duplication.  Add a comment describing the purpose
of the extracted code.  Fix coding style issues.