agent: adding KEYTOCARD command (master branch)

[NIIBE Yutaka]

In master branch, "keytocard" command for --edit-key is not supported
yet (the function g10/card_store_subkey).  I'd like to fix this.

I think that the reason why it is not yet supported is because of the
shift of secret key handling to gpg-agent.

Well, it is possible to implement this functionality again with
current gpg-agent's command set ("EXPORT_KEY" and "SCD"), but this is
not that good if we want to push secret key handling to gpg-agent.

Instead, I'd like to propose adding "KEYTOCARD" command to gpg-agent,
which directly implements the functionality of g10/card_store_subkey.
The command format would be like:

    KEYTOCARD [--force] <hexstring_with_keygrip> <keyid>
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