* Removed dependency from internal headers in rand.h. xmpp_rand
interface can be public in the future;
* xmpp_rand functions accept xmpp_rand_t object instead of xmpp_ctx_t.
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
