scd:p15: Match private keys with certificates also by labels.

* scd/app-p15.c (cdf_object_from_label): New.
(cdf_object_from_certid): Fallback to label matching.
(read_p15_info): Ditto.
(keygrip_from_prkdf): Ditto.  Replace duplicated code by a call to
cdf_object_from_objid.

In case there is no certificate for a private key we now also try to
find a certificate using a matching label.

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