gpg: Do not use a fixed string for --gpgconf-list:default_pubkey_algo.

* g10/keygen.c (get_default_pubkey_algo): New.
(parse_key_parameter_string): Use it.
* g10/gpg.c (gpgconf_list): Take value from new function.

Note that consumers of that gpgconf-list value may need to be adjusted
to that new value. It should anyway only be used to display the
default algorithm.

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