low risk, potentially varint overflow bug patched thanks to BBR
Removed ldns dependency

ldns dependency was only still around for constants defined in ldns/rr.h,
but those constants are RFC specified DNS constants, and to reduce deps
have been replicated in dns_utils.h instead of including ldns/rr.h.
Remove LDNS dep and fix a bug in libunbound const correctness fix
Merge pull request #160 from tewinget/remove_ldns_dep

Remove ldns dependency