agent: Add debug output for failed RSA signature verification

* agent/pksign.c (agent_pksign_do): Support ECC and DSA verification
and print some debug info in the error case.

Note that the addition of do_encode_dsa has no immediate effect
because we use the code only for RSA. However, for debugging it can
be useful to change the code to check also other created signatures.

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