NODEATH.DEH is a patch that makes all enemies dissappear when killed,
including the monsters on the last levels(though this can be changed easily
enough)  The net effect is that since the monsters don't technically "die",
you can run a game on NIGHTMARE in 1.2 and not have monsters respawn!
Finally, you can run UV speed and the registered version, with the same
effect as the -fast switch in 1.6!
