Space Invaders v1.1

Updated in 2001 to add the keyboard interrupt it was sorely needing. It plays much better than version 1.0.

Some other minor changes and fixes to the code and added the aliens from the Atari VCS version. Basic AdLib sound unchanged.

Tested and working 100% on DOSBox 0.74. Set cycles in the region of 90,000 for correct play speed.