sm: Print the serial number of a cert also in decimal.

* sm/certdump.c: Include membuf.h.
(gpgsm_print_serial_decimal): New.
* sm/keylist.c (list_cert_raw): Print s/n also in decimal
(list_cert_std): Ditto.

Many CA's print the serial number in decimal on their cards.

• Signed-off-by: Werner Koch <wk@gnupg.org>