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

@@ -130,6 +130,9 @@ func (l *SSHListener) Serve() {
 			term, err := l.handleConn(conn)
 
 			// Handshake is done (success or failure). Release limits.
+			// Explicit release is required because l.HandlerFunc below
+			// runs for the duration of the session. We only want to limit
+			// concurrent handshakes, not concurrent sessions.
 			release()
 
 			if err != nil {