g10,scd: Support single salt for KDF data object.

* g10/card-util.c (gen_kdf_data): Support single salt.
(kdf_setup): Can have argument for single salt.
* scd/app-openpgp.c (pin2hash_if_kdf): Support single salt.

Gnuk has "admin-less" mode. To support "admin-less" mode with KDF
feature, salt should be same for user and admin. Thus, I introduce a
valid use of single salt.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>