It listens on all addresses, which users might not want.

Updates #18806 (follow-up to feedback from CL 45088)

Change-Id: I51de2d3fc3cd88a61eb3c63018c47aea920c0549
Reviewed-on: https://go-review.googlesource.com/45157
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>