mod_external_services: Also validate services added by other modules This ensures that problems with entries added other modules, e.g. mod_turn_external, are reported on startup. However, this depends on load order and whether a `module:depends()` call comes before the `module:add_item()` call. A followup commit will do something about that.

local xpcall = xpcall;

if select(2, xpcall(function (x) return x end, function () end,  "test")) ~= "test" then
	xpcall = require"util.compat".xpcall;
end

return {
	xpcall = xpcall;
};