util: make GetDataDir read-only & create datadir..

.. only in bitcoind and bitcoin-qt

This changes behaviour of GetConfigFilePath which now always returns the
absolute path of the provided -conf argument.

src/qt/bitcoin.cpp

@@ -167,6 +167,7 @@ static void initTranslations(QTranslator &qtTranslatorBase, QTranslator &qtTrans
 
 static bool InitSettings()
 {
+    gArgs.EnsureDataDir();
     if (!gArgs.GetSettingsPath()) {
         return true; // Do nothing if settings file disabled.
     }