name space reasons. Was bignum_next_prime. Updated call in
rsa-keygen.c.
(primes): Use prime-list.h.
(nettle_next_prime): Skip Fermat test. Use mpz_millerrabin
directly, rather than mpz_probab_prime_p, when the former is
available.

Rev: nettle/bignum-next-prime.c:1.3