gpg: Allow unattended deletion of secret keys.

* agent/command.c (cmd_delete_key): Make the --force option depend on
--disallow-loopback-passphrase.
* g10/call-agent.c (agent_delete_key): Add arg FORCE.
* g10/delkey.c (do_delete_key): Pass opt.answer_yes to
agent_delete_key.

Unless the agent has been configured with
--disallow-loopback-passpharse an unattended deletion of a secret key
is now possible with gpg by using --batch _and_ --yes.

• Signed-off-by: Werner Koch <wk@gnupg.org>