net: permit use of Resolver.PreferGo, netgo on Windows and Plan 9

net: permit use of Resolver.PreferGo, netgo on Windows and Plan 9

This reverts commit CL 401754 (440c9312c8) which reverted CL 400654,
thus reapplying CL 400654, re-adding the func init() { netGo = true }
to cgo_stub.go CL 400654 had originally removed (mistakenly during
development?) that had broken the darwin nocgo builder.

Fixes #33097

Change-Id: I90f59746d2ceb6b5d2bd832c9fc90068f8ff7417
Reviewed-on: https://go-review.googlesource.com/c/go/+/409234
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Keith Randall <khr@google.com>