Add response code REFUSED at Unbound blocklists

[refuseduser]

Here is my PR for the addition of response code REFUSED at Unbound blocklists
Ref: #6027
Successfully tested on a live installation

https://github.com/opnsense/core/blob/master/src/opnsense/service/templates/OPNsense/Unbound/core/dnsbl_module.py

• Since I was already doing this anyway, I added RCODE_SERVFAIL as well as RCODE_REFUSED, as this was used several times but was not defined.
• Pyflake shows even more undefined variables here, e.g. RR_TYPE_A, but I haven't looked into it any further...

[AdSchellevis]

this needs a model migration (https://docs.opnsense.org/development/frontend/models_migrations.html#migrations). If you need help with that, we have to postpone the merge for a while (when I have some time to test this myself).