ipv6: document that package ipv6 doesn't support Darwin kernel version 12 or below

This change also uses nettest.SupportsIPv6MulticastDeliveryOnLoopback
to disable multicast packet delivery tests on some platform that doesn't
support the feature.

Fixes golang/go#17015.

Change-Id: Ia547efc2ac3779a96592bbc3874533fb65b711af
Reviewed-on: https://go-review.googlesource.com/33251
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed