two player mode now working

[Ulf Ekström]

Hi all. I have added two player support back into the game. The keys are now:
Player 1: Arrow keys+right ctrl
Player 2: w,s,a,d+space

Level 6 has two players in different colored planes. Support for
different plane models have been added to sprites/biplane.c. The models
are selected (at the moment) from the level file:

sprite{
	type = biplane
	variant = "red baron"
	ai = player2	
	...
}

The blue plane is called "blue duke" for now. The variant keyword
can easily be used also for other sprite types if we have any ideas.

Ulf