card: First code to actually create openpgp keys.

* tools/gpg-card.c (generate_all_openpgp_card_keys): Add demo key
generation.
(generate_key): Allow generatiing one OpenPGP key.

This does now allows to create a single OpenPGP key optioanlly with a
specified parameter. For example to create an auth key:

gpg-card generate --algo=ed25519 OPENPGP.3

Using option --force will overwrite and already existing key.
scdaemon does here take care of swicthing the key attributes before
generating the key.

TODO: We need to add some more stuff to app-openpgp so that the user
is not annoyed by beeing asked to enter the Admin-PIN twice (change
the key attributes clear the verification state). gpg's key
generation also needs some tweaks for using an existing card key which
has no key stub in private-keys-v1.d.

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