atomicfile: use ReplaceFile on Windows so that attributes and ACLs are preserved

I moved the actual rename into separate, GOOS-specific files. On
non-Windows, we do a simple os.Rename. On Windows, we first try
ReplaceFile with a fallback to os.Rename if the target file does
not exist.

ReplaceFile is the recommended way to rename the file in this use case,
as it preserves attributes and ACLs set on the target file.

Updates #14428

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

atomicfile/atomicfile_windows.go

@@ -0,0 +1,33 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package atomicfile
+
+import (
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+func rename(srcFile, destFile string) error {
+	// Use replaceFile when possible to preserve the original file's attributes and ACLs.
+	if err := replaceFile(destFile, srcFile); err == nil || err != windows.ERROR_FILE_NOT_FOUND {
+		return err
+	}
+	// destFile doesn't exist. Just do a normal rename.
+	return os.Rename(srcFile, destFile)
+}
+
+func replaceFile(destFile, srcFile string) error {
+	destFile16, err := windows.UTF16PtrFromString(destFile)
+	if err != nil {
+		return err
+	}
+
+	srcFile16, err := windows.UTF16PtrFromString(srcFile)
+	if err != nil {
+		return err
+	}
+
+	return replaceFileW(destFile16, srcFile16, nil, 0, nil, nil)
+}