scd: Let READKEY support --format=ssh option.
* scd/command.c (do_readkey): Support --format=ssh option. * common/ssh-utils.c (ssh_public_key_in_base64): New. * common/ssh-utils.h (ssh_public_key_in_base64): New declaration.
Code duplication (agent/command-ssh.c) will be cleaned up later.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>