Instead of hoping for the correct number of bits, fix the largest bit

[jgdumas]

This improves bn_gen_prime_factor, generating a prime with large (p-1) prime factor. Once the large prime factor, a, is found,

• the previous version:
  • tries to find primes p of the form 1 + a * (t+u) with random t and increasing u from 0
  • until p is prime and of the desired size
• the new version:
  • fixes t to be a power of two, and makes random selections of u's, so that p is almost always of the desired size
  • until p is prime and of the desired size
• the new version can be an order of magnitude faster on average, but also for the worst cases.