This change adds site-local unicast classification for users still using
the deprecated addresses internally.

Change-Id: If50870c6d4a85fe471c002b161eec59efcebe2f4
Reviewed-on: https://go-review.googlesource.com/12344
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>