When loading active items, increment the active_item_count.

Fixes #8121 and fixes #8491 they are basically the same. The active item counter has not been stored in the map files, but it has also not been recalculated upon loading the submap. So it started at 0 for each loaded submap.

This prevent the processing of active items there as the game thinks there are no active items to be processed on that submap.

This PR adds the proper check and increment of `active_item_count`, similar to what happens when an item is added (e.g. dropped) onto the map with `map::add_item`. It is compatible with existing saves.