agent: New GETINFO sub-command jent_active.

* agent/command.c (cmd_getinfo): Implement it for gcrypt >= 1.8.

For the de-vs compliance of gpg we need to check whether the Jitter
RNG is used on Windows. This change allows to test this for
gpg-agent.

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