* agent/command.c (cmd_learn): Allow for s/n argument.
* agent/learncard.c (agent_handle_learn): Ditto.
* agent/call-scd.c (agent_card_learn): Ditto.  Pass it on to scd.
* scd/command.c (cmd_switchcard): Factor most code out to ...
(switchcard_core): new.
(cmd_learn): Add option --demand to specify a s/n.
* sm/gpgsm.c (main): Allow a s/n argument for --learn-card.