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/connect_control.c

@@ -175,6 +175,7 @@ static struct command_result *json_connect(struct command *cmd,
 
 static const struct json_command connect_command = {
 	"connect",
+	"network",
 	json_connect,
 	"Connect to {id} at {host} (which can end in ':port' if not default). "
 	"{id} can also be of the form id@host"