unix: add Uname on FreeBSD

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>
diff --git a/unix/ztypes_freebsd_amd64.go b/unix/ztypes_freebsd_amd64.go
index dd0db2a..6580947 100644
--- a/unix/ztypes_freebsd_amd64.go
+++ b/unix/ztypes_freebsd_amd64.go
@@ -542,3 +542,11 @@
 type CapRights struct {
 	Rights [2]uint64
 }
+
+type Utsname struct {
+	Sysname  [256]byte
+	Nodename [256]byte
+	Release  [256]byte
+	Version  [256]byte
+	Machine  [256]byte
+}