scd:openpgp: Use shared fucntion for the dispserialno.
* scd/app-openpgp.c (wipe_and_free): New. (wipe_and_free_string): New. (get_disp_serialno): Remove. Replace callers by function app_get_dispserialno. (get_usage_string): New. (send_keypair_info): Use new function.
The new function has the same behaviour. The wipe functions are
not yet used.