[getdns-api] Rename dnssec_return_supporting_responses

[Willem Toorop]

The name dnssec_return_supporting_responses is a bit confusing because
the word "responses" seem to refer to actual response packets.  I
propose to rename this to dnssec_return_validation_chain.  The
"additional_dnssec" in the response dict should contain all the resource
record dicts for all the resource records (DS, DNSKEY and their RRSIGs)
that are needed to perform the validation from the root up.  The
resource record dicts should be in the same format as resource records
in the packet sections of replies in the "replies_tree" section of a
response dict.
What do you think?