This manually reverts 555da73c from #6372 which implies a
minimum FreeBSD version of 8-STABLE.
Updates docs to mention new minimum requirement.

Fixes #9627

Change-Id: I40ae64be3682d79dd55024e32581e3e5e2be8aa7
Reviewed-on: https://go-review.googlesource.com/3020Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>