QC, DOCS: Added infokey for disabling backpacks in deathmatch

[Zungrysoft]

Description of Changes

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I've always disliked the backpack mechanic in Quake 1 deathmatch. In duels, it punishes the out-of-control player for picking up ammo since often it will do little more than deliver the in-control player free ammo. I've added an infokey that lets you disable it. They're still enabled by default, of course.

Checklist

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• [x] I have read the LibreQuake contribution guidelines
• [x] I have thoroughly tested my changes to the best of my ability
• [x] I confirm I have not contributed anything that would impact LibreQuake's licensing and usage
• [ ] This Pull Request fixes a critical issue that should be reviewed and merged as soon as possible

[Nolcoz]

How does this work? Is it a command you have to put in console?

[Zungrysoft]

@Nolcoz QuakeWorld engines support infokeys, which are additional settings that change the way the game plays. The original QuakeWorld had infokeys like "dq", which controls whether or players drop the Quad Damage if they die while holding it or if it's just gone for good. I've added some new ones to let players customize their deathmatch play a bit more. They're all optional and must be enabled in the console. The console command to enable this particular infokey would be:

serverinfo disable_backpacks 1