json: Implement keylist

* src/gpgme-json.c (xjson_AddStringToObject0)
(xjson_AddItemToObject): New helpers.
(sig_notation_to_json, key_sig_to_json, tofu_to_json)
(uid_to_json, subkey_to_json, key_to_json): New
GPGME to JSON functions.
(op_keylist): New.
(process_request): Add op_keylist.

The conversion from GPGME data structures to
JSON follow the same pattern for the keylist
functions using the xjson wrappers instead
of error checking every cJSON call.

For large keylists the keylist command also
needs a data / getmore handling somehow.