crypto: Simplify asymetric key handling.

get_asymmetric_key() and free_asymmetric_key() are public because
para_server maintains a copy to the public key of each user so that
the keys need to be loaded only once. On the other hand, for private
keys (used in para_client) key allocation and freeing is performed
implicitly in priv_decrypt(), and no reference to the key is ever
returned. So the crypto API can be simplified by exposing the interface
only for public keys.

Hence this patch renames get_asymmetric_key() to get_public_key()
and drops the "private" argument. Similarly, free_asymmetric_key()
is renamed to free_public_key().