Refactor rand

* 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.

src/uuid.c

@@ -28,14 +28,14 @@
  */
 static void crypto_uuid_gen(xmpp_ctx_t *ctx, char *uuid)
 {
-    uint8_t buf[16];
+    unsigned char buf[16];
     int i = 0; /* uuid iterator */
     int j = 0; /* buf iterator */
 
     static const char hex[] = {'0', '1', '2', '3', '4', '5', '6', '7',
                                '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
 
-    xmpp_rand_bytes(ctx, buf, sizeof(buf));
+    xmpp_rand_bytes(ctx->rand, buf, sizeof(buf));
     buf[8] &= 0x3f;
     buf[8] |= 0x80;
     buf[6] &= 0x0f;