add stdin helpers for password input support

src/compat/stdin.h

@@ -0,0 +1,18 @@
+// Copyright (c) 2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_COMPAT_STDIN_H
+#define BITCOIN_COMPAT_STDIN_H
+
+struct NoechoInst {
+    NoechoInst();
+    ~NoechoInst();
+};
+
+#define NO_STDIN_ECHO() NoechoInst _no_echo
+
+bool StdinTerminal();
+bool StdinReady();
+
+#endif // BITCOIN_COMPAT_STDIN_H