gpg: Introduce magic value 25519 to switch a card to ECC.

* g10/card-util.c (ask_card_keyattr): Handle special value 25519.
(do_change_keyattr): Allow changing to cv25519/ed25519.
(generate_card_keys): Ditto.
(card_generate_subkey): Ditto.

This is kludge to make it easier for gnuk to be switched into ECC
mode. This is basically the same change as
commit ea09b6cded9d31a8ebd91878553c3eaa2b76e817
but without the string change in show_keysize_warning.

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