ecc: Change an error code of gcry_ecc_mul_point.

* cipher/ecc-ecdh.c (_gcry_ecc_mul_point): Return
GPG_ERR_UNKNOWN_CURVE.

Unknown_curve is more specific than unknown_algorithm.

This patch also adds documentation and renames rthe parameter from
'algo' to 'curveid'.

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