cmd/tailscaled: make state dir on Plan 9

Updates #5794

Change-Id: Id7bdc08263e98a1848ffce0dd25fc034747d7393
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -250,8 +250,19 @@ func main() {
 	// Only apply a default statepath when neither have been provided, so that a
 	// user may specify only --statedir if they wish.
 	if args.statepath == "" && args.statedir == "" {
+		if runtime.GOOS == "plan9" {
+			home, err := os.UserHomeDir()
+			if err != nil {
+				log.Fatalf("failed to get home directory: %v", err)
+			}
+			args.statedir = filepath.Join(home, "tailscale-state")
+			if err := os.MkdirAll(args.statedir, 0700); err != nil {
+				log.Fatalf("failed to create state directory: %v", err)
+			}
+		} else {
 			args.statepath = paths.DefaultTailscaledStateFile()
 		}
+	}
 
 	if args.disableLogs {
 		envknob.SetNoLogsNoSupport()