ec: Use BN_{CTX_,}secure_new memory API for priv keys

OpenSSL suggests to use (and internally itself uses)
`BN_{CTX_,}secure_new' primitives to work with private keys.

These are using `OPENSSL_secure_malloc' et al. calls, which use
special 'secure heap' memory.

Along, optimize out `hashsum2bn' with `BN_lebin2bn'.