hostinfo: fix earlier git fail, add files lost in move

This was meant to be part of 47045265b9
which instead deleted them :(

Updates tailscale/corp#1959

hostinfo/hostinfo_windows.go

@@ -0,0 +1,39 @@
+// Copyright (c) 2020 Tailscale Inc & 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 hostinfo
+
+import (
+	"os/exec"
+	"strings"
+	"sync/atomic"
+	"syscall"
+)
+
+func init() {
+	osVersion = osVersionWindows
+}
+
+var winVerCache atomic.Value // of string
+
+func osVersionWindows() string {
+	if s, ok := winVerCache.Load().(string); ok {
+		return s
+	}
+	cmd := exec.Command("cmd", "/c", "ver")
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+	out, _ := cmd.Output() // "\nMicrosoft Windows [Version 10.0.19041.388]\n\n"
+	s := strings.TrimSpace(string(out))
+	s = strings.TrimPrefix(s, "Microsoft Windows [")
+	s = strings.TrimSuffix(s, "]")
+
+	// "Version 10.x.y.z", with "Version" localized. Keep only stuff after the space.
+	if sp := strings.Index(s, " "); sp != -1 {
+		s = s[sp+1:]
+	}
+	if s != "" {
+		winVerCache.Store(s)
+	}
+	return s // "10.0.19041.388", ideally
+}