Add capability to +USE serverside ragdolls and toggle cleanup of individual ragdolls

[Blixibon]

This PR adds two new spawnflags to prop_ragdoll to enable +USE and prevent pickup, respectively. When a ragdoll has +USE enabled, the player will be able to grab a limb and drag it around. This uses the same framework as the mega physcannon, although pickup forces will only use the strength of the carried limb, rather than the strength of the entire bone hierarchy. The other spawnflag disables picking up the ragdoll even when +USE is enabled, and is intended for situations where a ragdoll should be useable but should not be picked up. An OnPlayerUse output is also included, and it fires every time the ragdoll is +USE'd regardless of whether it is picked up. A new cvar, ragdoll_always_allow_use, allows all ragdolls to be +USE'd regardless of the spawnflag, which may be useful if you're making a game or mod centered around this mechanic. This cvar is disabled by default.

This PR also includes two new inputs: AddToLRU and RemoveFromLRU. "LRU" stands for "Least Recently Used", and in this case it's referring to Source's ragdoll cleanup system, which automatically removes ragdolls which haven't been used. Serverside ragdolls spawned by NPCs are automatically added to this system, while prop_ragdolls spawned by the map are not. You can use the AddToLRU input to add any ragdoll to this system, or the RemoveFromLRU input to remove a ragdoll from this system. This is useful when you want a ragdoll created by a NPC to linger, or if you want a ragdoll created by the map to be treated as a regular NPC ragdoll.

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PR Checklist

• My PR follows all guidelines in the CONTRIBUTING.md file
• My PR targets a develop branch OR targets another branch with a specific goal in mind

[1upD]

Are you sure you want to add new spawnflags? The likelihood of Mapbase mods adding their own is low, but if they do then this update will be breaking.

[1upD]

I left a comment about changing spawnflags, but other than that everything looks good