Fix compile: missing #include <openssl/bn.h>
Fix compile: problems with defn of BN_zero

In OpenSSL 1.0.1j, BN_zero is defined in bn.h by

If OPENSSL_USE_DEPRECATED is not defined, then BN_zero returns void, not
int, so FAILZ(BN_zero((BIGNUM *)a0)) gives a compile error.
Fix compile: replace BN_init with BN_new

As of OpenSSL 1.1.0, `BN_init` is no longer available. Instead, use
BN_new to construct BIGNUMs.