document BN_GENCB API by adding an example

     functions operate on a caller-supplied key-structure and return

     success/failure rather than returning a key or NULL - this is to

     help make "keygen" another member function of RSA_METHOD etc.

     Example for using the new callback interface:

          int (*my_callback)(int a, int b, BN_GENCB *cb) = ...;

          void *my_arg = ...;

          BN_GENCB my_cb;

          BN_GENCB_set(&my_cb, my_callback, my_arg);

          return BN_is_prime_ex(some_bignum, BN_prime_checks, NULL, &cb);

          /* For the meaning of a, b in calls to my_callback(), see the

           * documentation of the function that calls the callback.

           * cb will point to my_cb; my_arg can be retrieved as cb->arg.

           * my_callback should return 1 if it wants BN_is_prime_ex()

           * to continue, or 0 to stop.

           */

     [Geoff Thorpe]

  *) Change the ZLIB compression method to be stateful, and make it