Fix unit attack pathing

This improves pathfinding in a bunch of cases relating to attacks: 

* Fixes #2104, #2923, #4449, #4455.
* Helicopters and planes can now Guard and Follow other actors (C&C only - requires AutoTargeting aircraft).
* Fixes a potential hang in Follow.
* Cleans up a bunch of crufty old code.

This will need a fair bit more testing, as it is fiddly code with a bunch of potential edge-cases.