The new implementation adds listening support on a multicast
address with plan9 network interfaces.

Fixes #17218

Change-Id: I2c75515e72e120acb71610cd077fddfbf9cf4e29
Reviewed-on: https://go-review.googlesource.com/29964Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>