card: Make "generate" work for PIV cards.

* tools/card-call-scd.c (scd_genkey_cb): Make createtime optional.
(scd_genkey_cb):  Ditto.  Add arg algo.
* tools/gpg-card-tool.c (cmd_generate): Add options and factor card
specific code out to ...
(generate_openpgp, generate_generic): new functions.

This patch keeps the interactive OpenPGP mode but adds a pure command
line mode for other cards; in particular PIV cards. What we still
need to do is:
a) Add an interactive mode for PIV cards
b) Add a command line mode for OpenPGP cards.

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