Fix 4801: silent NPE while Princess moving Wraith BA

[Sleet01]

This issue was caused by some confusion on my part when implementing proper Field Gun / Field Artillery expected damage computations for Princess.
The boolean fieldGunsDoDamage is actually checking if a unit can make any non-personal weapons attacks, because either:

1. the unit is infantry that hasn't moved this turn, or
2. the unit is literally any other thing.

The root cause was attempting to dereference the linked ammo for BA AP weapons, which doesn't work. That throws an NPE, which is caught in the movement path computation code, which returns a null mp object in that case. But then we try to use the null mp object anyhow, with predictable results.

Fix is to clean up some of the checks in that section of code:

• Rename the boolean for clarity
• Formally separate infantry with field weapons from other unit types in comments and code
• Simplify the field weapon expected damage check; we can let Princess assume only one shot from multi-fire field guns will hit, in which case we can use the rack value for all types instead of trying to compute expected damage more completely. This should also make Princess more likely to use infantry weapons in some cases.

Tested on the save file from the issue, as well as a custom save to verify other infantry and non-infantry predictions still work correctly.

Close #4801