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