Show command line options as dialog when opened from debug window

On Linux/Mac the command-line options were printed to stderr when the button
was pressed in the debug window, resulting in confusion. This is fixed
in this commit by adding a separate method.

src/qt/bitcoin.cpp

@@ -212,7 +212,7 @@ int main(int argc, char *argv[])
     if (mapArgs.count("-?") || mapArgs.count("--help"))
     {
         GUIUtil::HelpMessageBox help;
-        help.exec();
+        help.showOrPrint();
         return 1;
     }