Add options for corpse cleanup

[Lexicality]

Currently there's a 60s timer counting down that wipes everything when it runs
Instead, allow the same sort of options as weapons:

• Source Fade Corpses option (set on the platform itself which sets spawnflag 512 on created NPCs)
• No dropping corpses (delete corpses immediately on spawn)
• Delete corpses over x seconds old
• Only allow x dropped corpses (clean up old when new drops)
• Clean up all corpses option / API call (in cleanup - if it doesn't already exist)

[Lexicality]

I've just had a complaint about this, and it's fairly reasonable not to want this to happen as a global thing. As a first step, I should work out how to make it only affect spawn platform corpses.