Add POSSITEM source and credits, fix Demon Rifle altfire CTD bug

[Cacodemon345]

Also fixes a couple of other bugs.

[JMartinezHarmoniousPerson]

Bug report: Although the Demon Tech Rifle crash bug on overhauled enemies is fixed, however, where they're in possession state, shooting the Demon Tech Rifle will crash the game. I have got no crash log from it. Other weapons don't cause this.

[JMartinezHarmoniousPerson]

Update: It seems that other weapons are crashing as well when an overhauled enemy is at possession state. I'm wondering if this is related to the zombie_man_friendly actor too, as it seems to crash firing a weapon with that actor spawned too. Unless it's because of another infinite recursion from the possession state

No crash log as well

[Cacodemon345]

I frankly got no idea how to fix it properly.

[Cacodemon345]

I fixed it finally.

[JMartinezHarmoniousPerson]

Hey Cacodemon, have you checked my pull request to this PR yet? I have fixed several bugs in there and applied your Demon Tech Rifle crash bug fix to the Labguys (Labguys had their own void A_SmartChase() call due to them being a melee enemy).

[pa1nki113r]

Looks good. STRailgun has been renamed and revamped to just Railgun, other than that PBRifle and HMG have tiny conflicts that should be real quick to fix and it can be merged. Thanks for the life saving crash fix :)