FreeBSD doesn't have a uname syscall but the information can be
retrieved using sysctls the same way as on Darwin.

Change-Id: I824c42490d1feed3f1ad3823427c01dd3e5ea3c1
Reviewed-on: https://go-review.googlesource.com/79918
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>