Implement loadavg on all BSDs without cgo (#1584)

Reuse the Go-only implementation already in place for FreeBSD (#385) on
Darwin, DragonflyBSD, NetBSD and OpenBSD.

Tested on all affected platforms.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

collector/loadavg_freebsd.go

@@ -1,40 +0,0 @@
-// Copyright 2016 The Prometheus Authors
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// +build !noloadavg
-
-package collector
-
-import (
-	"unsafe"
-
-	"golang.org/x/sys/unix"
-)
-
-func getLoad() ([]float64, error) {
-	type loadavg struct {
-		load  [3]uint32
-		scale int
-	}
-	b, err := unix.SysctlRaw("vm.loadavg")
-	if err != nil {
-		return nil, err
-	}
-	load := *(*loadavg)(unsafe.Pointer((&b[0])))
-	scale := float64(load.scale)
-	return []float64{
-		float64(load.load[0]) / scale,
-		float64(load.load[1]) / scale,
-		float64(load.load[2]) / scale,
-	}, nil
-}