Bleichenbacher's DSA attack.  With this implementation, the expected
number of iterations never exceeds 2.

New semantics for BN_rand_range():
BN_rand_range(r, min, range) now generates r such that
     min <= r < min+range.
(Previously, BN_rand_range(r, min, max) generated r such that
     min <= r < max.
It is more convenient to have the range; also the previous
prototype was misleading because max was larger than
the actual maximum.)