Bitcoin-Qt: fixes for using display unit from options

- extend PaymentServer with setOptionsModel() and rework initNetManager()
  to make use of that
- fix all other places in the code to use display unit from options and no
  hard-coded unit

src/qt/bitcoingui.cpp

@@ -670,9 +670,12 @@ void BitcoinGUI::closeEvent(QCloseEvent *event)
 
 void BitcoinGUI::askFee(qint64 nFeeRequired, bool *payFee)
 {
+    if (!clientModel || !clientModel->getOptionsModel())
+        return;
+
     QString strMessage = tr("This transaction is over the size limit. You can still send it for a fee of %1, "
         "which goes to the nodes that process your transaction and helps to support the network. "
-        "Do you want to pay the fee?").arg(BitcoinUnits::formatWithUnit(BitcoinUnits::BTC, nFeeRequired));
+        "Do you want to pay the fee?").arg(BitcoinUnits::formatWithUnit(clientModel->getOptionsModel()->getDisplayUnit(), nFeeRequired));
     QMessageBox::StandardButton retval = QMessageBox::question(
           this, tr("Confirm transaction fee"), strMessage,
           QMessageBox::Yes|QMessageBox::Cancel, QMessageBox::Yes);