agent: Add known keys to sshcontrol.
* agent/command-ssh.c (ssh_identity_register): Add a key to sshcontrol even if it is already in the private key store. * tests/openpgp/ssh.scm: Test this.
- GnuPG-bug-id: T2316
- Signed-off-by: Justus Winter <justus@g10code.com>