Add -version option to get just the version

Adds a `-version` or `--version` option to print just the version
of the program for bitcoind, bitcoin-cli and bitcoin-qt.

Also make it that `-help` can be used to display the help (as well as
existing `--help`). Up to now, `-help` was the only option that didn't
work with either one or two dashes.

src/qt/bitcoingui.cpp

@@ -556,7 +556,7 @@ void BitcoinGUI::aboutClicked()
 
 void BitcoinGUI::showHelpMessageClicked()
 {
-    HelpMessageDialog *help = new HelpMessageDialog(this);
+    HelpMessageDialog *help = new HelpMessageDialog(this, false);
     help->setAttribute(Qt::WA_DeleteOnClose);
     help->show();
 }