scd:openpgp: Let the genkey function also accept a full keyref.

* scd/app-openpgp.c (send_key_attr): Use log_assert.
(do_genkey): Allow prefix.

It is more uniform to always use full keyref (e.g. "OPENPGP.1")
instead of just the key number.

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