Mitigate DoS by limiting concurrent handshakes

This commit introduces mechanism to limit the number of concurrent SSH
handshakes to prevent resource exhaustion (goroutines/memory) during a
SYN flood or connection flood attack.

1.  **Global Limit:** A semaphore limits total concurrent handshakes to 20.
    This applies backpressure to the `Accept` loop.
2.  **Per-IP Limit:** Using `github.com/sethvargo/go-limiter`, we now
    limit the rate of connection attempts per IP to 3 per second.
3.  **Timeout:** The initial handshake timeout is reduced from 20s to 10s.

The global semaphore is acquired before the handshake and released
immediately after the handshake completes (or fails), ensuring that
long-lived sessions do not consume a handshake slot.

sshd/net.go

@@ -92,7 +92,9 @@ func (l *SSHListener) Serve() {
 		host, _, err := net.SplitHostPort(conn.RemoteAddr().String())
 		if err != nil {
 			logger.Printf("Failed to split remote addr: %v", err)
-		} else {
+		}
+
+		if err == nil {
 			// Context with timeout is not strictly needed for memory store, but good practice
 			// Although Take is non-blocking for memory store usually.
 			_, _, _, ok, err := l.limiter.Take(context.Background(), host)