ecc: Add key generation flag "no-keytest".

* src/cipher.h (PUBKEY_FLAG_NO_KEYTEST): New.
* cipher/pubkey-util.c (_gcry_pk_util_parse_flaglist): Add flag
"no-keytest".  Return an error for invalid flags of length 10.
* cipher/ecc.c (nist_generate_key): Replace arg random_level by flags
set random level depending on flags.
* cipher/ecc-eddsa.c (_gcry_ecc_eddsa_genkey): Ditto.
* cipher/ecc.c (ecc_generate): Pass flags to generate fucntion and
remove var random_level.
(nist_generate_key): Implement "no-keytest" flag.
* tests/keygen.c (check_ecc_keys): Add tests for transient-key and
no-keytest.

After key creation we usually run a test to check whether the keys
really work. However for transient keys this might be too time
consuming and given that a failed test would anyway abort the process
the optional use of a flag to skip the test is appropriate.

Using Ed25519 for EdDSA and the "no-keytest" flags halves the time to
create such a key. This was measured by looping the last test from
check_ecc_keys() 1000 times with and without the flag.

Due to a bug in the flags parser unknown flags with a length of 10
characters were not detected. Thus the "no-keytest" flag can be
employed by all software even for libraries before this. That bug is
however solved with this version.

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