control/controlclient: use API to get Windows version number

Fixes #1478

Signed-off-by: Aleksandar Pesic <peske.nis@gmail.com>

control/controlclient/hostinfo_windows_test.go

@@ -0,0 +1,28 @@
+// 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])
+	}
+}