ecc: Put the curve name again into the output of gcry_pk_genkey.

* cipher/ecc.c (ecc_generate): Use the correct var.  Release
CURVE_FLAGS.

Signed-off-by: Werner Koch <wk@gnupg.org>
mpi: Do not clear the sign of the mpi_mod result.

* mpi/mpi-mod.c (_gcry_mpi_mod): Remove sign setting.

Signed-off-by: Werner Koch <wk@gnupg.org>
tests: Add test options to keygen.

* tests/keygen.c (usage): New.
(main): Print usage info.  Allow running just one algo.

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