agent: Use SETDATA --apend for larger data to communicate scdaemon.
* agent/call-scd.c (prepare_setdata): New. (agent_card_pksign): Use prepare_setdata for SETDATA. (agent_card_pkdecrypt): Likewise.
- GnuPG-bug-id: T7436
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>