agent: Support --ack option for POPUPPINPADPROMPT.

* agent/divert-scd.c (getpin_cb): Support --ack option.

We are now introducing "acknowledge button" feature to scdaemon,
so that we can support OpenPGPcard User Interaction Flag.

We will (re)use the mechanism of POPUPPINPADPROMPT for this. Perhaps,
we will change the name of POPUPPINPADPROMPT, since it will be no
longer for PINPAD only.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>