pc_cp2.wad MAP38 completion fix

[Player701]

MAP38 of 2048 Unleashed (pc_cp2.wad) cannot be completed in single player because it relies on a hidden Commander Keen (Thing 101) to call A_KeenDie to open some doors, but the Keen actor is heavily modified by DEHACKED and does not call that anymore (and does not even die in the first place due to much higher health).

This PR fixes the issue by replacing the offending Keen with an imp and setting its special to emulate the effects of A_KeenDie (open doors tagged 666). This allows the gameplay to progress normally so that the map can now be finished.

This map is rather short, so I've played through it from start to finish and made sure it actually works now. If further verification is necessary, please follow these instructions after starting the map:

1. First, go straight ahead for a bit and then turn left, you will see a lightning bolt symbol.
2. Shoot the symbol to reveal a switch, then press the switch.
3. After about 40 seconds, the four pillars in the corners of this room in will open, one of these pillars reveals a switch that allows you to progress further. (Without the fix, the pillars do not open and you get stuck.)

[Player701]

A minor inconsistency though: the monster I replaced the Keen with is not an imp but actually a zombieman (it still does work all the same, though). I guess I was for some reason thinking of an imp because I mentioned it in my previous PR. Could someone please fix the comment?

Upd: submitted a follow-up fix, see #2318