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/ui_interface.h

@@ -63,6 +63,9 @@ public:
         /** Force blocking, modal message box dialog (not just OS notification) */
         MODAL               = 0x10000000U,
 
+        /** Do not print contents of message to debug log */
+        SECURE              = 0x40000000U,
+
         /** Predefined combinations for certain default usage cases */
         MSG_INFORMATION = ICON_INFORMATION,
         MSG_WARNING = (ICON_WARNING | BTN_OK | MODAL),