jsonrpc: Add a category field to commands.

A new string field is added to the command structure and is specified at the creation of each native command, and in the JSON created by 'json_add_help_command()'.

lightningd/log.c

@@ -779,6 +779,7 @@ static struct command_result *json_getlog(struct command *cmd,
 
 static const struct json_command getlog_command = {
 	"getlog",
+	"utility",
 	json_getlog,
 	"Show logs, with optional log {level} (info|unusual|debug|io)"
 };