Display security state when asking for password

Indicate to the user how secure the transport channel is so they
can avoid entering their password for untrusted sites.
diff --git a/common/rfb/CSecurityStack.h b/common/rfb/CSecurityStack.h
index a76b3fe..a16003f 100644
--- a/common/rfb/CSecurityStack.h
+++ b/common/rfb/CSecurityStack.h
@@ -32,6 +32,7 @@
     virtual bool processMsg(CConnection* cc);
     virtual int getType() const {return type;};
     virtual const char* description() const {return name;}
+    virtual bool isSecure() const;
   protected:
     int state;
     CSecurity* state0;