ai_helper.get_closest_location(): fix bug with tags in SLF

The way how the Standard Location Filter was dealt with only worked
with keys, not tags.
Messenger MAI: fix bug with escort units blocking the messenger

When there were lots of escort units, they previously quite frequently
blocked the messenger’s way, preventing him from making progress even
when it was possible.  Escort units now specifically move out of the
way if they do not need to attack enemy units in the messenger’s way.