unix: add SysctlTimeval on *bsd and darwin

Change-Id: I18daa353d101e6636a732288c8276d9f68d25f90
Reviewed-on: https://go-review.googlesource.com/c/sys/+/244237
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/syscall_bsd_test.go

@@ -10,6 +10,7 @@ import (
 	"os/exec"
 	"runtime"
 	"testing"
+	"time"
 
 	"golang.org/x/sys/unix"
 )
@@ -73,3 +74,11 @@ func TestSysctlClockinfo(t *testing.T) {
 	t.Logf("tick = %v, hz = %v, profhz = %v, stathz = %v",
 		ci.Tick, ci.Hz, ci.Profhz, ci.Stathz)
 }
+
+func TestSysctlTimeval(t *testing.T) {
+	tv, err := unix.SysctlTimeval("kern.boottime")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("boottime = %v", time.Unix(tv.Unix()))
+}