ipn, paths: unconditionally attempt to set state dir perms, but only if the state dir is ours

We unconditionally set appropriate perms on the statefile dir.

We look at the basename of the statefile dir, and if it is "tailscale", then
we set perms as appropriate.

Fixes #2925
Updates #2856

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

paths/paths_windows.go

@@ -6,6 +6,8 @@ package paths
 
 import (
 	"os"
+	"path/filepath"
+	"strings"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
@@ -86,6 +88,9 @@ func ensureStateDirPerms(dirPath string) error {
 	if !fi.IsDir() {
 		return os.ErrInvalid
 	}
+	if strings.ToLower(filepath.Base(dirPath)) != "tailscale" {
+		return nil
+	}
 
 	// We need the info for our current user as SIDs
 	sids, err := getCurrentUserSids()