[MP] Fixes for NPCs in multiplayer

[Blixibon]

Is your feature request related to a problem? Please describe.

Source SDK 2013's multiplayer does not support NPCs in virtually any capacity. The code relies on UTIL_GetLocalPlayer() and AI_GetSinglePlayer() too much, often not accounting for when either of the two don't exist.

Describe the solution you'd like

Partial integration of SecobMod is the most common and probably the best solution in this case. Some of the more subjective features (e.g. the portal gun) are not needed, but the actual fixes are exactly what seem to be needed.

Describe alternatives you've considered

SecobMod's NPC fixes originate from Winston's AI Multiplayer patch, which could probably be used on its own instead, although I don't know the differences.

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Additional context

SecobMod's repo is here, although there's also a Steam discussion with a newer patch.

[Blixibon]

Partial SecobMod integration is almost certain at this point, especially since there's a few other features (e.g. restoring singleplayer HL2 features) which seem interesting for giving mappers control over.