net: make go DNS use localhost if resolv.conf is missing or empty

net: make go DNS use localhost if resolv.conf is missing or empty

Per resolv.conf man page, "If this file does not exist, only the name
server on the local machine will be queried."

This behavior also occurs if file is present but unreadable,
or if no nameservers are listed.

Fixes #10566

Change-Id: Id5716da0eae534d5ebfafea111bbc657f302e307
Reviewed-on: https://go-review.googlesource.com/9380
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>