Add -walletdir parameter to specify custom wallet dir

src/wallet/walletutil.h

@@ -0,0 +1,13 @@
+// Copyright (c) 2017 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_WALLET_UTIL_H
+#define BITCOIN_WALLET_UTIL_H
+
+#include "util.h"
+
+//! Get the path of the wallet directory.
+fs::path GetWalletDir();
+
+#endif // BITCOIN_WALLET_UTIL_H