* agent/cvt-openpgp.c (try_do_unprotect_arg_s): Add field "curve".
(get_keygrip): Add and use arg CURVE.
(convert_secret_key): Ditto.
(convert_transfer_key): Ditto.
(get_npkey_nskey): New.
(prepare_unprotect): Replace gcrypt functions by
get_npkey_nskey.  Allow opaque MPIs.
(do_unprotect): Use CURVE instead of parameters.
(convert_from_openpgp_main): Ditto.
(convert_to_openpgp):  Simplify.
* g10/import.c (one_mpi_from_pkey): Remove.
(transfer_secret_keys): Rewrite to use the curve instead of the
parameters.
* g10/parse-packet.c (parse_key): Mark protected MPIs with USER1 flag.
* common/openpgp-oid.c (openpgp_curve_to_oid): Allow the use of
 "NIST P-256" et al.
* g10/keygen.c (ask_curve): Add arg ALGO.
(generate_keypair): Rewrite the ECC key logic.
* tests/openpgp/ecc.test: Provide the "ecc" passphrase.