init: stop allocating reg table functions with properties

This partly reverts commit f1844decef because a user reported seeing
nil passed as the object to the reg.update() function.

init.lua

@@ -152,12 +152,11 @@ function vicious.register(widget, wtype, format, timer, warg)
         timer  = timer,
         warg   = warg,
         widget = widget,
-
-        -- Update function
-        update = function ()
-            update(widget, reg)
-        end,
     }
+    -- Set functions
+    reg.update = function ()
+        update(widget, reg)
+    end
 
     -- Default to 2s timer
     if reg.timer == nil then