gpg: Allow "futuredefault" as alias for "future-default".

* g10/keygen.c (parse_key_parameter_string): Allow "futuredefault" and
use case-insensitive matching
(quick_generate_keypair): Ditto.
(parse_algo_usage_expire): Ditto.

The man page is sometimes rendered in a way that the hyphen may be
not be considered as part of the string. And while at it we also
allow case-insensitivity.

• GnuPG-bug-id: T3655
• Signed-off-by: Werner Koch <wk@gnupg.org>