net: fix Dial(":80") on Windows

Windows sockets allow bind to 0.0.0.0:80 but not connect to it.
To make Listen(":80") / Dial(":80") work as documented on Windows,
connect to 127.0.0.1 or ::1 (depending on network) in place of 0.0.0.0.

Fixes #6290.

Change-Id: Ia27537067276871648546678fbe0f1b8478329fe
Reviewed-on: https://go-review.googlesource.com/32101


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>