Extract MakeUnique into utilmemory.h

And use it to reduce chainparamsbase's direct reliance on util.h to
only args handling.

utilmemory.h can be replaced with <memory> once we move to C++14.

src/utilmemory.h

@@ -0,0 +1,19 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2018 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_UTILMEMORY_H
+#define BITCOIN_UTILMEMORY_H
+
+#include <memory>
+#include <utility>
+
+//! Substitute for C++14 std::make_unique.
+template <typename T, typename... Args>
+std::unique_ptr<T> MakeUnique(Args&&... args)
+{
+    return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
+}
+
+#endif