lib: Load built-in Lua functions

lib: Load built-in Lua functions

We can't use even `string()` function because built-in functions are not
loaded.

Testing with:

```
$ cat /etc/frr/scripts/zebra.lua
function on_rib_process_dplane_results(ctx)
	log.warn(string.upper("testas"))
	return {}
end
```

This results to "TESTAS" in the logs.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>