gpg: Implement card_store_subkey again.
* g10/call-agent.h (agent_keytocard): New. * g10/call-agent.c (agent_keytocard): New. * g10/card-util.c (replace_existing_key_p): Returns 1 when replace. (card_generate_subkey): Check return value of replace_existing_key_p. (card_store_subkey): Implement again using agent_keytocard.