Unix and Windows differ in how they handle LookupCNAME(name).
If name exists in DNS with an A or AAAA record but no CNAME,
then on all operating systems and in the pure Go resolver,
LookupCNAME returns the name associated with the A/AAAA record
(the original name).
TestLookupCNAME has been testing this, because www.google.com
has no CNAME. I think it did at one point, but not anymore, and the
tests are still passing. Also added google.com as a test, since
top-level domains are disallowed from having CNAMEs.
If name exists in DNS with a CNAME record pointing to a record that
does not exist or that has no A or AAAA record,
then Windows has always reported the CNAME value,
but Unix-based systems and the pure Go resolver have reported
a failure instead. cname-to-txt.go4.org is an test record that is
a CNAME to a non-A/AAAA target (the target only has a TXT record).
This CL changes the Unix-based systems and the pure Go resolver
to match the Windows behavior, allowing LookupCNAME to succeed
whenever a CNAME exists.
TestLookupCNAME nows tests the new behavior by looking up
cname-to-txt.go4.org (run by bradfitz).