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/CSecurityTLS.h b/common/rfb/CSecurityTLS.h
index 57d964d..e726d1e 100644
--- a/common/rfb/CSecurityTLS.h
+++ b/common/rfb/CSecurityTLS.h
@@ -48,6 +48,7 @@
     virtual int getType() const { return anon ? secTypeTLSNone : secTypeX509None; }
     virtual const char* description() const
       { return anon ? "TLS Encryption without VncAuth" : "X509 Encryption without VncAuth"; }
+    virtual bool isSecure() const { return !anon; }
     static void setDefaults();
 
     static StringParameter X509CA;