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/mksyscall.go

@@ -0,0 +1,7 @@
+// Copyright 2009 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.
+
+package windows
+
+//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -output zsyscall_windows.go eventlog.go service.go syscall_windows.go security_windows.go