scripted-diff: Avoid interface keyword to fix windows gitian build

Rename `interface` to `interfaces`

Build failure reported by Chun Kuan Lee <ken2812221@gmail.com>
https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-379434756

-BEGIN VERIFY SCRIPT-

git mv src/interface src/interfaces
ren() { git grep -l "$1" | xargs sed -i "s,$1,$2,g"; }
ren interface/            interfaces/
ren interface::           interfaces::
ren BITCOIN_INTERFACE_    BITCOIN_INTERFACES_
ren "namespace interface" "namespace interfaces"

-END VERIFY SCRIPT-

src/interfaces/handler.cpp

@@ -0,0 +1,33 @@
+// 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.
+
+#include <interfaces/handler.h>
+
+#include <util.h>
+
+#include <boost/signals2/connection.hpp>
+#include <memory>
+#include <utility>
+
+namespace interfaces {
+namespace {
+
+class HandlerImpl : public Handler
+{
+public:
+    HandlerImpl(boost::signals2::connection connection) : m_connection(std::move(connection)) {}
+
+    void disconnect() override { m_connection.disconnect(); }
+
+    boost::signals2::scoped_connection m_connection;
+};
+
+} // namespace
+
+std::unique_ptr<Handler> MakeHandler(boost::signals2::connection connection)
+{
+    return MakeUnique<HandlerImpl>(std::move(connection));
+}
+
+} // namespace interfaces