agent: Support --ack option for POPUPPINPADPROMPT.

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

Cherry-picked master commit of:
827529339a4854886dbb5625238e7e01013efdcd

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>