gpg: Improve the new parse_subkey_algostr_usagestr fucntion.
* g10/keygen.c (parse_usagestr): Allow "cert". (generate_subkeypair): Factor expire parsing out to ... (parse_subkey_algostr_usagestr): here. Rename to ... (parse_algo_usage_expire): this. Add arg 'for_subkey'. Set CERT for primary key and check that it is not set for subkeys.
- Signed-off-by: Werner Koch <wk@gnupg.org>