mpz_import is available, define nettle_mpz_from_octets as a macro
calling mpz_import.
(nettle_mpz_from_octets): Start by setting x to zero; callers no
longer need to do that.
(nettle_mpz_set_str_256_s): New logic for the handling of negative
numbers. Convert in the same way as for positive numbers, and then
subtract the appropriate power of two.

Rev: src/nettle/bignum.c:1.9