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/common.h

@@ -27,6 +27,7 @@
 #include "hash.h"
 #include "util.h"
 #include "parser.h"
+#include "rand.h"
 
 /** run-time context **/
 
@@ -45,6 +46,7 @@ struct _xmpp_ctx_t {
     const xmpp_mem_t *mem;
     const xmpp_log_t *log;
 
+    xmpp_rand_t *rand;
     xmpp_loop_status_t loop_status;
     xmpp_connlist_t *connlist;
 };