Pull BIGNUM constants out of get_* function bodies

Pull BIGNUM constants out of get_* function bodies

The get_rfc*_prime_* functions will be removed. The constants will remain
for the BN_get_rfc*_prime_* functions. Make the latter call BN_bin2bn()
directly on these constants rather than going through get_*. This avoids
some overlong lines. Also KNF for some comments.

Reduces the diff I currently carry by quite a bit.