Deal with table.pack corner-cases properly (#10350)

c.f., https://github.com/koreader/koreader-base/pull/1603 for more
details.

Re: #9624

doc/Events.md

@@ -41,7 +41,7 @@ for _, widget in ipairs(self) do
     end
 end
 -- If not consumed by children, consume it ourself
-return self["on"..event.name](self, unpack(event.args))
+return self["on"..event.name](self, unpack(event.args, 1, event.args.n))
 ```
 
 ## Event system

frontend/dbg.lua

@@ -49,7 +49,7 @@ function Dbg:turnOn()
             if post_guard then
                 post_guard(...)
             end
-            return unpack(values)
+            return unpack(values, 1, values.n)
         end
     end
     --- Use this instead of a regular Lua @{assert}().

frontend/ui/trapper.lua

@@ -674,7 +674,7 @@ function Trapper:dismissableRunInSubprocess(task, trap_widget_or_string, task_re
         if task_returns_simple_string then
             return completed, ret_values
         else
-            return completed, unpack(ret_values)
+            return completed, unpack(ret_values, 1, ret_values.n)
         end
     end
     return completed

frontend/ui/uimanager.lua

@@ -361,7 +361,7 @@ function UIManager:debounce(seconds, immediate, action)
         else
             is_scheduled = false
             if not immediate then
-                result = action(unpack(args))
+                result = action(unpack(args, 1, args.n))
             end
             if not is_scheduled then
                 -- This check is needed because action can recursively call debounced_action_wrapper
@@ -376,7 +376,7 @@ function UIManager:debounce(seconds, immediate, action)
             self:scheduleIn(seconds, scheduled_action)
             is_scheduled = true
             if immediate then
-                result = action(unpack(args))
+                result = action(unpack(args, 1, args.n))
             end
         end
         return result
@@ -987,7 +987,7 @@ function UIManager:_checkTasks()
             -- NOTE: Said task's action might modify _task_queue.
             --       To avoid race conditions and catch new upcoming tasks during this call,
             --       we repeatedly check the head of the queue (c.f., #1758).
-            task.action(unpack(task.args))
+            task.action(unpack(task.args, 1, task.args.n))
         else
             -- As the queue is sorted in descending order, it's safe to assume all items are currently future tasks.
             wait_until = task_time

frontend/ui/widget/eventlistener.lua

@@ -34,7 +34,7 @@ By default, it's `"on"..Event.name`.
 function EventListener:handleEvent(event)
     if self[event.handler] then
         --print("EventListener:handleEvent:", event.handler, "handled by", debug.getinfo(self[event.handler], "S").short_src, self)
-        return self[event.handler](self, unpack(event.args))
+        return self[event.handler](self, unpack(event.args, 1, event.args.n))
     end
 end