scd: Use Gcrypt usage constants for the do_with_keygrip capabilities.

* scd/command.c (cmd_keyinfo): Use Gcrypt constants for CAP.
* scd/app-openpgp.c (do_with_keygrip): Adjust for them.
* scd/app-piv.c (do_with_keygrip): Ditto.

That makes it easier to read. An open question is whether we should
allow several capabilities and whether they are the ORed or ANDed.

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