agent: PKSIGN should return signature in same format for card.
* agent/pksign.c (agent_pksign_do):
It's best to keep same data format by libgcrypt.
For card (due to historical reasons), gpg-agent or scdaemon used to
prefix 0x00 when it starts 0x80, so that it can be parsed signed MPI
as well as unsigned MPI. It used to do nothing for preceding zeros.
- Signed-off-by: NIIBE Yutaka <firstname.lastname@example.org>