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/env_windows.go

@@ -0,0 +1,29 @@
+// Copyright 2010 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.
+
+// Windows environment variables.
+
+package windows
+
+import "syscall"
+
+func Getenv(key string) (value string, found bool) {
+	return syscall.Getenv(key)
+}
+
+func Setenv(key, value string) error {
+	return syscall.Setenv(key, value)
+}
+
+func Clearenv() {
+	syscall.Clearenv()
+}
+
+func Environ() []string {
+	return syscall.Environ()
+}
+
+func Unsetenv(key string) error {
+	return syscall.Unsetenv(key)
+}