sodium_init() will return 0 on success, -1 on failure, and 1 if sodium is
already loaded and initialized (which is not an error). In the case
where libsodium is already initialized and the system is restarted we
may return 1 from onload nif function resulting in a crash.

- change the call to sodium_init() to check for an error return (-1) and
  return -1 explicitly in this case, otherwise always return zero at the
  end of our onload function.

Fix NIF-load issue on init:restart

Merge upstream

on Darwin allow platform to decide arch

This adds a low-level APIs to perform computations over the edwards25519
curve, only useful to implement custom constructions.

Exposed functions:
  crypto_ed25519_scalarmult/2,
  crypto_ed25519_scalarmult_base/1,
  crypto_ed25519_scalarmult_noclamp/2,
  crypto_ed25519_scalarmult_base_noclamp/1,
  crypto_ed25519_add/2,
  crypto_ed25519_sub/2,
  crypto_ed25519_is_valid_point/1,
  crypto_ed25519_scalar_reduce/1,
  crypto_ed25519_scalar_negate/1,
  crypto_ed25519_scalar_add/2,
  crypto_ed25519_scalar_sub/2,
  crypto_ed25519_scalar_mul/2