botan: Encode curve OID and public key in EC private key

botan: Encode curve OID and public key in EC private key

Without OID we can't generate an algorithmIdentifier when loading the
key again. And older versions of OpenSSL insist on a public key when
e.g. converting a key to PKCS#8.

Simply unwrapping the ECPrivateKey structure avoids log messages when
parsing other keys in the KEY_ANY case.