Revert "control/controlclient: use API to get Windows version number"

This reverts commit 4224b3f731.

From https://github.com/tailscale/tailscale/pull/1494#discussion_r594852889 ...

> Actually, I want all four numbers back. I spent the evening
> debugging an issue for a user running an old version of Windows
> and then going to to
> https://en.wikipedia.org/wiki/Windows_10_version_history_(version_1809)
> and reading all the revision notes in the footnotes of that wikipedia
> page.
>
> I'm going to revert this for now for Tailscale 1.6. We can land it
> again later when we figure out how to get the fourth numbers.

Updates #1478

control/controlclient/hostinfo_windows.go

@@ -5,9 +5,9 @@
 package controlclient
 
 import (
-	"fmt"
-
-	"golang.org/x/sys/windows"
+	"os/exec"
+	"strings"
+	"syscall"
 )
 
 func init() {
@@ -15,6 +15,16 @@ func init() {
 }
 
 func osVersionWindows() string {
-	major, minor, build := windows.RtlGetNtVersionNumbers()
-	return fmt.Sprintf("%d.%d.%d", major, minor, build)
+	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:]
+	}
+	return s // "10.0.19041.388", ideally
 }

control/controlclient/hostinfo_windows_test.go

@@ -1,28 +0,0 @@
-// Copyright (c) 2021 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 controlclient
-
-import (
-	"os/exec"
-	"regexp"
-	"testing"
-)
-
-func TestOsVersionWindows(t *testing.T) {
-	out, err := exec.Command("cmd", "/c", "ver").Output()
-	if err != nil {
-		t.Fatalf("`ver` error: %v", err)
-	}
-	// Extract the version number from the output, and isolate the first three parts (major.minor.build)
-	rx := regexp.MustCompile(`(\d+\.\d+\.\d+)(\.\d+)?`)
-	m := rx.FindStringSubmatch(string(out))
-	if m == nil {
-		t.Fatalf("no version number in `ver` output: %q", out)
-	}
-	got := osVersionWindows()
-	if m[1] != got {
-		t.Errorf("osVersionWindows got %q want %q", got, m[1])
-	}
-}