Accuracy improvments imported from Kex :D

r1275 | svkaiser | 2014-02-25 21:04:12 -0800 (Tue, 25 Feb 2014)

+ New linedef flag (NOMONSTERTRIGGER) discovered. Fixes bug with Imps
activating the secret exit door in Spawned Fear
+ Major code change to collision logic in order to properly emulator key
bump/grabbing
+ Line trigger/shooting behavior more accurate to original