Generate a new prime number of prime_bits bits and store it in prime. If factor_bits is non-zero, one of the prime factors of (prime - 1) / 2 must be factor_bits bits long. If factors is non-zero, allocate a new,
NULL-terminated array holding the prime factors and store it in factors. flags might be used to influence the prime number generation process.
Find a generator for prime where the factorization of (prime-1) is in the
NULLterminated array factors. Return the generator as a newly allocated MPI in r_g. If start_g is not NULL, use this as the start for the search.