openssl.c: clean up curve25519 code (#499)

File: openssl.c, openssl.h, crypto.h, kex.c

Notes:
This cleans up a few things in the curve25519 implementation:

- There is no need to create X509_PUBKEYs or PKCS8_PRIV_KEY_INFOs to
  extract key material. EVP_PKEY_get_raw_private_key and
  EVP_PKEY_get_raw_public_key work fine.

- libssh2_x25519_ctx was never used (and occasionally mis-typedefed to
  libssh2_ed25519_ctx). Remove it. The _libssh2_curve25519_new and
  _libssh2_curve25519_gen_k interfaces use the bytes. Note, if it needs
  to be added back, there is no need to roundtrip through
  EVP_PKEY_new_raw_private_key. EVP_PKEY_keygen already generated an
  EVP_PKEY.

- Add some missing error checks.

Credit:
David Benjamin

src/crypto.h

@@ -181,8 +181,8 @@ _libssh2_ecdsa_curve_type_from_name(const char *name,
 #if LIBSSH2_ED25519
 
 int
-_libssh2_curve25519_new(LIBSSH2_SESSION *session, libssh2_ed25519_ctx **ctx,
-                        uint8_t **out_public_key, uint8_t **out_private_key);
+_libssh2_curve25519_new(LIBSSH2_SESSION *session, uint8_t **out_public_key,
+                        uint8_t **out_private_key);
 
 int
 _libssh2_curve25519_gen_k(_libssh2_bn **k,