Add ncl_restore_cvars_values stock

[Polarhigh]

It would be helpful to have such a stock:

stock ncl_restore_cvars_values(id)
{
    ncl_sandbox_cvar_begin(id);

    for (new eSandboxCvar:i; i < eSandboxCvar; i++) {
        ncl_write_sandbox_cvar(i, "");
    }

    ncl_sandbox_cvar_end();
}