Minor update to fortresses

The conquerable fortresses across Arda just got an overhaul. No change in their
general behaviour should be observable (yet), except for the following:

- Former defenders and mounts that now follow their conquerors are not
  considered part of the garrison anymore.
- Fresh garrisons will not take control of a fort occupied by Zaugurz Orcs.

Please report any bugs.

-Waba.