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_bsd.go

@@ -11,6 +11,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build darwin dragonfly freebsd netbsd openbsd
 // +build !noloadavg
 
 package collector

collector/loadavg_unix.go

@@ -1,33 +0,0 @@
-// Copyright 2015 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 darwin dragonfly netbsd openbsd
-// +build !noloadavg
-
-package collector
-
-import (
-	"errors"
-)
-
-// #include <stdlib.h>
-import "C"
-
-func getLoad() ([]float64, error) {
-	var loadavg [3]C.double
-	samples := C.getloadavg(&loadavg[0], 3)
-	if samples != 3 {
-		return nil, errors.New("failed to get load average")
-	}
-	return []float64{float64(loadavg[0]), float64(loadavg[1]), float64(loadavg[2])}, nil
-}