agent: Fix agent_public_key_from_file for ECC.

* agent/cvt-openpgp.c (extract_private_key): New.
(convert_to_openpgp): Use extract_private_key.
* agent/findkey.c (agent_public_key_from_file): Use
extract_private_key.

This patch add support of ECC key with a curve name and flags. Since
same functionality is also needed for convert_to_openpgp, it was
factored out into the extract_private_key function.