json: Add direct way to query a config option

* src/gpgme-json.c (op_config_opt, hlp_config_opt): New operation.
(process_request, hlp_help): Add it.

--
This is more conveniant API for most query operations where
a single option is required.

An example would be:
{
    "op": "config_opt",
    "component": "gpg",
    "option": "default-key"
}

Which results in:
{
   "option":       {
           "name": "default-key",
           "description":  "use NAME as default secret key",
           "argname":      "NAME",
           "flags":        0,
           "level":        0,
           "type": 1,
           "alt_type":     1,
           "value":        [{
                           "string":       "F462B6B1",
                           "is_none":      false
                   }]
   }
}

src/gpgme-json.c

@@ -2460,6 +2460,102 @@ leave:
 }
 
 
+static const char hlp_config_opt[] =
+  "op:       \"config_opt\"\n"
+  "component: The component of the option.\n"
+  "option:    The name of the option.\n"
+  "\n"
+  "Response on success:\n"
+  "\n"
+  "option: Information about the option.\n"
+  " String values:\n"
+  "  name: The name of the option\n"
+  "  description: Localized description of the opt.\n"
+  "  argname: Thhe argument name e.g. --verbose\n"
+  "  default_description\n"
+  "  no_arg_description\n"
+  " Number values:\n"
+  "  flags: Flags for this option.\n"
+  "  level: the level of the description. See gpgme_conf_level_t.\n"
+  "  type: The type of the option. See gpgme_conf_type_t.\n"
+  "  alt_type: Alternate type of the option. See gpgme_conf_type_t\n"
+  " Arg type values: (see desc. below)\n"
+  "  default_value: Array of the default value.\n"
+  "  no_arg_value: Array of the value if it is not set.\n"
+  "  value: Array for the current value if the option is set.\n"
+  "\n"
+  "If the response is empty the option was not found\n"
+  "";
+static gpg_error_t
+op_config_opt (cjson_t request, cjson_t result)
+{
+  gpg_error_t err;
+  gpgme_ctx_t ctx = NULL;
+  gpgme_conf_comp_t conf = NULL;
+  gpgme_conf_comp_t comp = NULL;
+  cjson_t j_tmp;
+  char *comp_name = NULL;
+  char *opt_name = NULL;
+
+  ctx = get_context (GPGME_PROTOCOL_GPGCONF);
+
+  j_tmp = cJSON_GetObjectItem (request, "component");
+  if (!j_tmp || !cjson_is_string (j_tmp))
+    {
+      err = gpg_error (GPG_ERR_INV_VALUE);
+      goto leave;
+    }
+  comp_name = j_tmp->valuestring;
+
+
+  j_tmp = cJSON_GetObjectItem (request, "option");
+  if (!j_tmp || !cjson_is_string (j_tmp))
+    {
+      err = gpg_error (GPG_ERR_INV_VALUE);
+      goto leave;
+    }
+  opt_name = j_tmp->valuestring;
+
+  /* Load the config */
+  err = gpgme_op_conf_load (ctx, &conf);
+  if (err)
+    {
+      goto leave;
+    }
+
+  comp = conf;
+  for (comp = conf; comp; comp = comp->next)
+    {
+      gpgme_conf_opt_t opt = NULL;
+      int found = 0;
+      if (!comp->name || strcmp (comp->name, comp_name))
+        {
+          /* Skip components if a single one is specified */
+          continue;
+        }
+      for (opt = comp->options; opt; opt = opt->next)
+        {
+          if (!opt->name || strcmp (opt->name, opt_name))
+            {
+              /* Skip components if a single one is specified */
+              continue;
+            }
+          xjson_AddItemToObject (result, "option", conf_opt_to_json (opt));
+          found = 1;
+          break;
+        }
+      if (found)
+        break;
+    }
+
+leave:
+  gpgme_conf_release (conf);
+  release_context (ctx);
+
+  return err;
+}
+
+
 static const char hlp_config[] =
   "op:     \"config\"\n"
   "\n"
@@ -2689,6 +2785,7 @@ process_request (const char *request)
     const char * const helpstr;
   } optbl[] = {
     { "config",     op_config,     hlp_config },
+    { "config_opt", op_config_opt, hlp_config_opt },
     { "encrypt",    op_encrypt,    hlp_encrypt },
     { "export",     op_export,     hlp_export },
     { "decrypt",    op_decrypt,    hlp_decrypt },