Corrected `guid` implementation of item package in 51074a0

* GUID
  - `guid` field in official PackageItem.lua supposed to make item will be separated even item is stackable item.
  - Currently `guid` usage is using item Unique ID that generated when item with GUID flag is appear from package.
  - Some item containers in official PackageItem.lua are also separated, not stacked when player obtain it. Example when using atcommand "@item Xmas_Bless". For current implementation, the items are flagged in item_flag.txt (db folder) with flag 4.
  - `isNamed` field in item group is replaced by `GUID` and move the `isNamed` as new additional field. See doc/item_group.txt or db/re/item_package.txt.
  - Added core.h config `ENABLE_ITEM_GUID` (b92b8ff)
    - Enabled by default to take effect of `guid` function.
    - When it's disabled, `guid` function will be ignored as suggestion to disable this 'weird. official feature.
    - NOTE: If this feature is disabled "in the middle" of your game, the separated is still separated in inventory, storage, or guild storage until player move the item to/from storage/inventory to restack them.
* NPC:
  - Enabled npc/re/other/item_merge.txt and it's now usable.
* Script Command:
  - Added `mergeitem` to merge separated items in player's inventory. See doc/script_commands.txt.
* Misc:
  - Changed how to broadcast the item that obtained from package with flag `isAnnounced` to intif_broadcast, so it will be announced to all connected map-servers.
  - Added some items that flagged as 'item group container' in db/re/item_flag.txt

Signed-off-by: Cydh Ramdh <house.bad@gmail.com>
This command will merge all items with the given item ID or name that is flagged as GUID item type (flag 4) in the item_flag database.