agent: Return NO_SECKEY instead of ENONET for PKSIGN and others.
* agent/pksign.c (agent_pksign_do): Replace ENONET by NO_SECKEY. * agent/findkey.c (agent_key_from_file): No diagnostic for NO_SECKEY. * agent/pkdecrypt.c (agent_pkdecrypt): Replace checking for ENOENT.