CL 32101 fixed Dial(":80") on Windows and added TestDialLocal,
which was failing on Plan 9, because it wasn't implemented
on Plan 9.

This change implements Dial(":80") by connecting to 127.0.0.1
or ::1 (depending on network), so it works as documented.

Fixes #17760.

Change-Id: If0ff769299e09bebce11fc3708639c1d8c96c280
Reviewed-on: https://go-review.googlesource.com/32593Reviewed-by: Russ Cox <rsc@golang.org>