scd: Better user interaction for factory-reset.

* g10/card-util.c (factory_reset): Dummy PIN size is now 32-byte.
Connect the card again at the last step.

Before the change, a user has to quit the session to continue. Now,
it is possible to type RET in the session and see if it's really done.

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