ecc: Improve gcry_pk_get_curve.

* cipher/ecc-curves.c (_gcry_ecc_fill_in_curve): Factor some code out
to ..
(find_domain_parms_idx): new.
(_gcry_ecc_get_curve): Find by curve name on error.

This change allows the use of an input with just the curve name which
can be used to test whether a given curve has been implemented. Is is
required because due to the "param" flag change the caller usually
does not have the key parameters available.

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