Add nonce plugin implementation

This nonce generator uses an RNG to generate nonces. The RNG quality is
currently set to RNG_WEAK which is the same value used in IKE init.

The plugin is enabled and thus built by default.

src/libstrongswan/plugins/nonce/nonce_plugin.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2012 Adrian-Ken Rueegsegger
+ * Hochschule fuer Technik Rapperswil
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.  See <http://www.fsf.org/copyleft/gpl.txt>.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * for more details.
+ */
+
+/**
+ * @defgroup nonce_p nonce
+ * @ingroup plugins
+ *
+ * @defgroup nonce_plugin nonce_plugin
+ * @{ @ingroup nonce_p
+ */
+
+#ifndef NONCE_PLUGIN_H_
+#define NONCE_PLUGIN_H_
+
+#include <plugins/plugin.h>
+
+typedef struct nonce_plugin_t nonce_plugin_t;
+
+/**
+ * Plugin implementing a nonce generator using an RNG.
+ */
+struct nonce_plugin_t {
+
+	/**
+	 * Implements plugin interface
+	 */
+	plugin_t plugin;
+};
+
+#endif /** NONCE_PLUGIN_H_ @}*/