void libscrypt_PBKDF2_SHA256(const uint8_t *, size_t, const uint8_t *, size_t, uint64_t, uint8_t *, size_t)
PBKDF2_SHA256(passwd, passwdlen, salt, saltlen, c, buf, dkLen): Compute PBKDF2(passwd, salt, c, dkLen) using HMAC-SHA256 as the PRF, and write the output to buf.