Enable Invasion in Multiplayer / Coop

[cfprovg]

After having exhausted the possibilities of using sav to map tricks to enable Invasion in multiplayer, I was hoping that it may be possible for the Crusader Patch to add this feature to the game as strongholds AI has always lackluster when it comes to providing a challenge to experienced players and a coop invasion mode would be present an excellent and highly customizable challenge for stronghold veterans.

[gynt]

Partial duplicate of #214

I think this is indeed feasible to do.