* agent.h: Add a callback function to the pin_entry_info structure.
* query.c (agent_askpin): Use the callback to check for a correct
PIN.  Removed the start_err_text argument becuase it is not
anymore needed; changed callers.
* findkey.c (unprotect): Replace our own check loop by a callback.
(try_unprotect_cb): New.
* genkey.c (reenter_compare_cb): New.
(agent_genkey): Use this callback here.  Fixed setting of the pi2
variable and a segv in case of an empty PIN.
* divert-scd.c (getpin_cb): Removed some unused stuff and
explained what we still have to change.