scd: Add option --info to emit KEYPAIRINFO by readkey command.

* scd/command.c (do_readkey): Implement this.
* scd/app-help.c (app_help_get_keygrip_string_pk): Make HEXKEYGRIP
parm optional.  Add arg R_ALGOSTR.

This patch basically mimics what we do in 2.3.

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