json: Add subkey_algo and defaults to createkey

* src/gpgme-json.c (op_createkey, hlp_createkey): Add subkey_algo
handling.
(hlp_createkey): Fix documentation of expiry.

Due to the funny quick-gen-key interface generating a key
with an explicit algo would result in bad defaults (only an SC key),
without a subkey.

This adds handling that should probably be in GnuPG proper to fix
the semantics of createkey by adding default subkey_algo handling.