Store negative cache entries under their natural type

Store negative cache entries under their natural type

Negative cache entries were exposed through dns_rdataset_t in an
inverted shape (type=none, covers=<qtype>) while the cache internally
stored them under (<qtype>, none) with a NEGATIVE flag, so every
consumer had to convert between the two representations, and several
places relied on type==0 as an implicit negativity test.

Expose negative entries in their natural shape instead: type holds the
RR type whose nonexistence is cached (dns_rdatatype_any for NXDOMAIN
and NODATA(QTYPE=ANY) proofs), covers stays none, and the negative
attribute marks the entry.  The implicit type==0 tests become explicit
attribute checks, and the cache database now rejects meta-types other
than ANY, which remains valid as a lookup type and as a negative
entry.