logpolicy: on Windows, use tailscale-ipn log name if it already existed

For the migration to tailscaled.exe on Windows, don't create a new logid
if one existed under the old filename.

Updates #1232

logpolicy/logpolicy.go

@@ -339,6 +339,18 @@ func New(collection string) *Policy {
 	tryFixLogStateLocation(dir, cmdName)
 
 	cfgPath := filepath.Join(dir, fmt.Sprintf("%s.log.conf", cmdName))
+
+	// The Windows service previously ran as tailscale-ipn.exe, so
+	// let's keep using that log base name if it exists.
+	if runtime.GOOS == "windows" && cmdName == "tailscaled" {
+		const oldCmdName = "tailscale-ipn"
+		oldPath := filepath.Join(dir, oldCmdName+".log.conf")
+		if fi, err := os.Stat(oldPath); err == nil && fi.Mode().IsRegular() {
+			cfgPath = oldPath
+			cmdName = oldCmdName
+		}
+	}
+
 	var oldc *Config
 	data, err := ioutil.ReadFile(cfgPath)
 	if err != nil {