agent: pksign result conversion to sexp to upper layer.

* agent/agent.h (divert_pksign): Add R_SIGLEN argument.
* agent/divert-scd.c (divert_pksign): Return length at R_SIGLEN.
* agent/call-scd.c (agent_card_pksign): Move composition of
S-expression to...
* agent/pksign.c (agent_pksign_do): ... here.
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Composing S-expression would be better to be done by SCDaemon.