agent: RSA signature verification by gpg-agent.

* g10/sign.c (do_sign): Let verify signature by gpg-agent.
* agent/pksign.c (agent_pksign_do): Call gcry_pk_verify for RSA.

RSA signature verification should be done to prevent attacks against
RSA CRT implementations and not to return invalid signature to
adversary. Newer libgcrypt does so. For older libgcrypt and
smartcards, gpg-agent does signature verification.