Add a SECURE style flag for ThreadSafeMessageBox, which indicates that the message contains sensitive information. This keeps the message from being output to the debug log by bitcoind. Fixes a possible security risk when starting bitcoind in server mode without the 'rpcpassword' option configured, resulting in the "suggested" password being output to the debug log.

src/rpcserver.cpp

@@ -581,7 +581,7 @@ void StartRPCThreads()
                 strWhatAmI,
                 GetConfigFile().string(),
                 EncodeBase58(&rand_pwd[0],&rand_pwd[0]+32)),
-                "", CClientUIInterface::MSG_ERROR);
+                "", CClientUIInterface::MSG_ERROR | CClientUIInterface::SECURE);
         StartShutdown();
         return;
     }