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/ctx.c

@@ -404,6 +404,11 @@ xmpp_ctx_t *xmpp_ctx_new(const xmpp_mem_t * const mem,
 
 	ctx->connlist = NULL;
 	ctx->loop_status = XMPP_LOOP_NOTSTARTED;
+	ctx->rand = xmpp_rand_new(ctx);
+	if (ctx->rand == NULL) {
+	    xmpp_free(ctx, ctx);
+	    ctx = NULL;
+	}
     }
 
     return ctx;
@@ -418,6 +423,7 @@ xmpp_ctx_t *xmpp_ctx_new(const xmpp_mem_t * const mem,
 void xmpp_ctx_free(xmpp_ctx_t * const ctx)
 {
     /* mem and log are owned by their suppliers */
+    xmpp_rand_free(ctx, ctx->rand);
     xmpp_free(ctx, ctx); /* pull the hole in after us */
 }