core: Support exporting secret keys.
* src/export.c (export_start): Set command handler for passphrase interaction for the case when it's secret keys. (export_ext_start): Likewise.
- GnuPG-bug-id: T5046
- Co-authored-by: Louis Dupré Bertoni
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>