mysql: Recognize when client/server negotiate SSL

This instantiates the SSL analyzer when the client requests SSL
so that Zeek now has a bit more visibility into encrypted MySQL
connections.

The pattern used is the same as in the IMAP, POP or XMPP analyzer.

src/analyzer/protocol/mysql/MySQL.h

@@ -25,11 +25,14 @@ public:
 	// Overridden from analyzer::tcp::TCP_ApplicationAnalyzer.
 	void EndpointEOF(bool is_orig) override;
 
+	void StartTLS();
+
 	static analyzer::Analyzer* Instantiate(Connection* conn) { return new MySQL_Analyzer(conn); }
 
 protected:
 	binpac::MySQL::MySQL_Conn* interp;
 	bool had_gap;
+	bool tls_active;
 	};
 
 	} // namespace zeek::analyzer::mysql