Add pidfile support

Copy a simple implementation from the Moby project, since importing
their package would pull in too many dependencies.

Fixes #320

vendor/golang.org/x/sys/windows/aliases.go

@@ -0,0 +1,13 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build windows
+// +build go1.9
+
+package windows
+
+import "syscall"
+
+type Errno = syscall.Errno
+type SysProcAttr = syscall.SysProcAttr