gpg: Make card key generation work again.

* g10/call-agent.c (agent_scd_learn): Rename from agent_learn.
(agent_learn): New.
* g10/keygen.c (gen_card_key): Call new agent-learn.

Without a shadow key we can't create the self-signatures. Thus we
need to issue the learn command after each key generation.

• Signed-off-by: Werner Koch <wk@gnupg.org>