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.h

@@ -0,0 +1,35 @@
+// 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_INTERFACES_HANDLER_H
+#define BITCOIN_INTERFACES_HANDLER_H
+
+#include <memory>
+
+namespace boost {
+namespace signals2 {
+class connection;
+} // namespace signals2
+} // namespace boost
+
+namespace interfaces {
+
+//! Generic interface for managing an event handler or callback function
+//! registered with another interface. Has a single disconnect method to cancel
+//! the registration and prevent any future notifications.
+class Handler
+{
+public:
+    virtual ~Handler() {}
+
+    //! Disconnect the handler.
+    virtual void disconnect() = 0;
+};
+
+//! Return handler wrapping a boost signal connection.
+std::unique_ptr<Handler> MakeHandler(boost::signals2::connection connection);
+
+} // namespace interfaces
+
+#endif // BITCOIN_INTERFACES_HANDLER_H