Add mxs::extract_sql

The helper function extracts the SQL contents as std::string from a
buffer. This should make it easier to log the query in C++ code.

include/maxscale/modutil.hh

@@ -0,0 +1,29 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+/**
+ * @file modutil.hh C++ additions/alternatives for modutil.h functions
+ */
+
+#include <maxscale/cppdefs.hh>
+#include <maxscale/modutil.h>
+
+#include <string>
+
+namespace maxscale
+{
+
+std::string extract_sql(GWBUF* buffer, size_t len = -1);
+
+}