Introduced PRNG based on Hash_DRBG (NIST SP 800-90A)

This pseudo-random number generator solves problem with
platform-independent generation of randomized nonces.
Current implementation uses weak entropy, especially when
kernel.randomize_va_space = 0. But it can be improved by
adding new sources to xmpp_rand_reseed().

New internal API introduced:
  xmpp_rand_new
  xmpp_rand_free
  xmpp_rand
  xmpp_rand_bytes
  xmpp_rand_nonce

src/rand.h

@@ -0,0 +1,43 @@
+/* rand.h
+ * strophe XMPP client library -- pseudo-random number generator
+ *
+ * Copyright (C) 2014 Dmitry Podgorny <pasis.ua@gmail.com>
+ *
+ *  This software is provided AS-IS with no warranty, either express
+ *  or implied.
+ *
+ *  This software is distributed under license and may not be copied,
+ *  modified or distributed except as expressly authorized under the
+ *  terms of the license contained in the file LICENSE.txt in this
+ *  distribution.
+ */
+
+/** @file
+ *  Pseudo-random number generator.
+ */
+
+#ifndef __LIBSTROPHE_RAND_H__
+#define __LIBSTROPHE_RAND_H__
+
+#include "strophe.h"
+#include "ostypes.h"
+
+typedef struct _xmpp_rand_t xmpp_rand_t;
+
+xmpp_rand_t *xmpp_rand_new(xmpp_ctx_t *ctx);
+void xmpp_rand_free(xmpp_ctx_t *ctx, xmpp_rand_t *rand);
+
+/** Analogue of rand(3). */
+int xmpp_rand(xmpp_ctx_t *ctx);
+
+/** Generates random bytes. */
+void xmpp_rand_bytes(xmpp_ctx_t *ctx, uint8_t *output, size_t len);
+
+/** Generates a nonce that is printable randomized string.
+ *
+ * @param len Number of bytes reserved for the output string, including
+ *            end of line '\0'.
+ */
+void xmpp_rand_nonce(xmpp_ctx_t *ctx, char *output, size_t len);
+
+#endif /* __LIBSTROPHE_RAND_H__ */