gpg: Make sure that we are not accidently working with the PIV app.

* g10/call-agent.c (agent_scd_switchapp): New.
* g10/card-util.c (get_info_for_key_operation): Call it.

It may happen that the active card was last used for PIV and in that
case certain commands will fail because they assume the OpenPGP app.
Fortunately we have a pretty central place to assure that the right
app has been selected.

The bug can be easily noticed on Windows.

• GnuPG-bug-id: T6378