Stop using 'arg' for varargs (it's removed in Lua 5.2/Wesnoth 1.11)

('arg' for varargs is also deprecated and partly broken in Lua 5.1:
https://stackoverflow.com/questions/18679174/why-does-the-use-of-in-any-expression-in-a-function-cause-the-value-of-arg-t
)