Fix missing RRSIGs for "glue" lookups with CD=1

Fix missing RRSIGs for "glue" lookups with CD=1

The code to test whether to store the RRSIGs on DNS_R_UNCHANGED
with CD=1 was failing because the comparison methods of the two
rdatatset instances were not compatible.  Move the testing into
dns_db_addrdataset(), and request it by setting the DNS_ADD_EQUALOK
option.  If the option is set and the old and new rrsets compare
as equal, dns_db_addrdataset() returns ISC_R_SUCCESS instead of
DNS_R_UNCHANGED.