Move FreeBSD/DragonflyBSD out of meminfo add kvm. (#547)

* Move FreeBSD/DragonflyBSD out of meminfo add kvm.

This gives us SwapUsed, and everything under one roof.

* Fix typos per review.

* Update to use newer API.

* Remove premature optimization per PR feedback.

collector/kvm_bsd.h

@@ -0,0 +1,19 @@
+// Copyright 2017 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 !nomeminfo
+// +build freebsd dragonfly
+
+#include <sys/types.h>
+
+int _kvm_swap_used_pages(uint64_t *out);