- Implement SIGINT signal handler for both wssshc and wsssht
- Add graceful shutdown logic that sends tunnel_close messages for active tunnels
- Implement 3-second timeout for tunnel close operations to complete
- Add double Ctrl+C detection for immediate exit without waiting
- Update signal handling throughout both programs to use graceful_shutdown flag

This ensures that active tunnels are properly closed when the programs
exit, either normally or via Ctrl+C, with a timeout mechanism and
fallback for immediate exit on second Ctrl+C.