fix: IW3 & T4 restrict VM shutdown handlers to game script shutdowns

[regenerance]

Title

fix: IW3 & T4 restrict VM shutdown handlers to game script shutdowns

Description

Fixes IW3 & T4 VM shutdown handling so plugin-owned script state is only cleared when the game script system is actually being torn down.

Previously, vmShutdownHandlers were executed for every Scr_ShutdownSystem call regardless of the sys value. IW3 & T4 uses this value to identify different script/string-system shutdown categories, including temporary and zone-related cleanup.

Temporary shutdowns occur during normal gametype initialization and transitions. Treating these as a complete VM shutdown caused persistent script state, such as replacefunc mappings, to be cleared prematurely.

The shutdown hook now executes VM shutdown handlers only for SCRIPT_USER_GAME a.k.a. sys == 1 for IW3 and bComplete on T4.

This prevents temporary shutdowns from incorrectly clearing game VM state.

Changes

• Restrict vmShutdownHandlers to SCRIPT_USER_GAME shutdowns.
• Preserve plugin-owned VM state during temporary script-system cleanup.
• Prevent replacefunc mappings and other VM-owned state from being cleared during normal gametype initialization.