openssl: abstract out OPENSSL_cleanse

This makes it easier for us to replace it if desired, since it's now only in
one spot. Also, it avoids the openssl include from allocators.h, which
essentially forced openssl to be included from every compilation unit.

src/support/cleanse.cpp

@@ -0,0 +1,13 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2015 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 "cleanse.h"
+
+#include <openssl/crypto.h>
+
+void memory_cleanse(void *ptr, size_t len)
+{
+    OPENSSL_cleanse(ptr, len);
+}