scd:p15: New flag APP_LEARN_FLAG_REREAD.

* scd/app-p15.c (do_deinit): Factor code out to ...
(release_lists, release_tokeninfo): new.
(read_ef_tokeninfo): Reset all data before reading.
(read_p15_info): Ditto.
(do_learn_status): Implement reread flag.

Objects of a card may be protected by a PIN and thus only readble
after a verify command. This new flag makes it possible to gather the
entire PKCS#15 card info again without a need for a reset (and thus
loss of the verify status).

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