fix handling of errors resolving one of paired A+AAAA query

fix handling of errors resolving one of paired A+AAAA query

the old logic here likely dates back, at least in inspiration, to
before it was recognized that transient errors must not be allowed to
reflect the contents of successful results and must be reported to the
application.

here, the dns backend for getaddrinfo, when performing a paired query
for v4 and v6 addresses, accepted results for one address family even
if the other timed out. (the __res_msend backend does not propagate
error rcodes back to the caller, but continues to retry until timeout,
so other error conditions were not actually possible.)

this patch moves the checks to take place before answer parsing, and
performs them for each answer rather than only the answer to the first
query. if nxdomain is seen it's assumed to apply to both queries since
that's how dns semantics work.