net: don't return DNS query results including the second best records unconditionally

This change prevents DNS query results using domain search list
overtaking results not using the list unconditionally, which only
happens when using builtin DNS stub resolver.

The previous internal lookup function lookup is split into lookup and
goLookupIPOrder for iteration over a set of names: FQDN or absolute
FQDN, with domain label suffixes in search list, without domain label
suffixes, and for concurrent A and AAAA record queries.

Fixes #11081.

Change-Id: I9ff0640f69276e372d97e709b149ed5b153e8601
Reviewed-on: https://go-review.googlesource.com/10836


Reviewed-by: Russ Cox <rsc@golang.org>