g10: Fix keytocard.

g10/call-agent.h (agent_scd_learn): Add FORCE option.
g10/call-agent.c (agent_scd_learn): Implement FORCE option.
g10/keygen.c (gen_card_key): Follow the change of option.
g10/card-util.c (change_pin, card_status, factory_reset): Likewise.
g10/keyedit.c (keyedit_menu): Update private key storage by

agent_scd_learn.

This is not a perfect solution since there is a possibility user
unplug card before quitting 'gpg --keyedit' session. Usually,
it works well.

• GnuPG-bug-id: T1846