minimise-any: prefer popular and not large RRset

[hdais]

NSD4's RFC8482 (minimizing response for ANY query) just picks one rrset element in linked list with no preference. This can pick a large rrset (e.g. DNSKEY, RRSIG) for response, but this is not suitable for RFC8482 purpose. For example dig @h.root-servers.net . ANY often gets all RRSIGs of ".". (H-root adopts NSD4).

This patch introduces simple preference between rtypes on selecting a rrset. It prefers popular and not large rtypes (A, AAAA,..), and lowers large rtypes (DNSKEY, RRSIG,..).

Preferece order is:

1. "Preferred" rtypes (A, AAAA, MX, SOA, PTR)
2. "Normal" rtypes (other than 1 and 3)
3. "Lowered" rtypes (DNSKEY, RRSIG, NSEC)

[wcawijngaards]

This change looks good to me, for the purpose of making smaller answers.

[wcawijngaards]

Thanks for the patch! This looks like it would make even smaller replies for type ANY. I have merged it in the code.