Remove the negative type logic from qpcache

Remove the negative type logic from qpcache

Previously, when a negative header was stored in the cache, it would be
stored in the dns_typepair_t as .type = 0, .covers = <negative type>.
When searching the cache internally, we would have to look for both
positive and negative typepair and the slabheader .down list could be a
mix of positive and negative types.

Remove the extra representation of the negative type and simply use the
negative attribute on the slabheader.  Other units (namely dns_ncache)
can still insert the (0, type) negative rdatasets into the cache, but
internally, those will be converted into (type, 0) slabheaders, and vice
versa - when binding the rdatasets, the negative (type, 0) slabheader
will be converted to (0, type) rdataset.  Simple DNS_TYPEPAIR() helper
macro was added to simplify converting single rdatatype to typepair
value.

As a side-effect, the search logic in all places can exit early if
there's a negative header for the type we are looking for, f.e. when
searching for the zone cut, we don't have to walk through all the
slabheaders, if there's a stored negative slabheader.