Extended item_check to also add unique_ids to any non-stackable items that lack it
- This is useful in case an old server that didn't have the unique_id feature enabled is updated to the latest version.
- It'll only have effect if item_check is enabled in the configuration files.
Fixed an issue when Token_Of_Siegfried is used, under certain conditions
- If the Token_Of_Siegfried is the first item in inventory, it would not get detected and consumed correctly.
Follow-up to b294026e6614a652c23bb0cea8a0d4dc69d8b125

- Added documentation for the pc->search_item function and formalized
  its return values. If the searched item is not found, now it returns
  the newly introduced constant INDEX_NOT_FOUND.
- Updated pc->search_item checks to make use of INDEX_NOT_FOUND.
- Fixed an issue with anvils not detected by the weapon forginc code,
  if they were in the first position of the inventory.
- Added ITEMID constants for the four anvil types.

Signed-off-by: Haru <haru@dotalux.com>