awful.util.join: Stop using arg

Implementing vararg functions via arg is deprecated in lua. This kind of thing
should instead be done via "...".

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/util.lua.in

@@ -237,9 +237,9 @@ end
 -- @return A new table containing all keys from the arguments.
 function table.join(...)
     local ret = {}
-    for i = 1, arg.n do
-        if arg[i] then
-            for k, v in pairs(arg[i]) do
+    for i, t in ipairs({...}) do
+        if t then
+            for k, v in pairs(t) do
                 if type(k) == "number" then
                     rtable.insert(ret, v)
                 else