ecc: Fix bug in gcry_pk_get_param for Curve25519.

* cipher/ecc-curves.c (_gcry_ecc_get_param_sexp): Simplify.
* cipher/pubkey.c (map_algo): Also map EDDSA to ECC.
* tests/curves.c (check_get_params): Add simple param lookup tests by
for all curves.

There is no actual need to compute affine ccordinates given that we
already got them in the curve parameter table. Thus most code could
be removed and thus won't run into the problem getting an Y coordinate
for a Montgomery curve.

EDDSA was not mapped to ECC: The map function was written a year
before EDDSA and I obviously forgot to add it.

• GnuPG-bug-id: T5490
• Signed-off-by: Werner Koch <wk@gnupg.org>