gpg: Change API of agent_scd_serialno.
* g10/call-agent.c (agent_scd_serialno): Extend API to allow with R_SERIALNO == NULL. * g10/card-util.c (card_status): Use NULL for agent_scd_serialno. (factory_reset): Likewise. * g10/skclist.c (build_sk_list): Likewise.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>