(_nettle_generate_pocklington_prime): New

argument top_bits_set, to optionally generate primes with the two
most significant bits set. Reordered argument list.
(nettle_random_prime): Likewise, added top_bits_set argument.
Invoke progress callback when a prime is generated.

Rev: nettle/bignum-random-prime.c:1.6
Rev: nettle/bignum.h:1.7