implement restart_level

[Paril]

Support for restart_level is required for modern co-op to work properly

[Paril]

notes on implementation:

• I added this to Kex to fix a common complaint about Quake II's savegame system: transitioning between levels overwrites 'current', and you essentially lose the state that you had at the start of the level, especially if you then switch to a different campaign
• Q2PRO does have a split between autosave and level start save already; it does diverge from kex but not in a way that causes problems (in Q2PRO, save1 is used for autosave and save0 is used for entering)
• in Kex, mid-level saves (both autosave and manual saves) create an additional set of files in the current slot for restart support; they are .restart.game.ssv, .restart.server.ssv and .restart.sv2, respectively. They are copied from game.ssv, server.ssv and <sv.save_name>.sv2 before WriteGame/WriteLevel is run, and if they already exist they are not overridden.
• If a level transition occurs, those three restart files are deleted.
• These special files cannot be loaded manually in any way other than restart_level.
• Running restart_level will initiate a "soft reload"; it first checks if the .restart files exist in the slot. If not, it will use the slot itself (because the only way this would happen is if you started a map and didn't do any saves yet, so 'restarting' the level is the same as just loading the slot directly).
• Technically, one could implement this system without it being a soft reload, and just do the regular load stuff. The downside is this takes longer and requires clients to reconnect.
• Kex's soft reload performs a soft wipe of the server by doing the following:
  • backing up entity client numbers
  • copying configstrings to temp memory
  • reading server file
  • clearing world
  • call SpawnEntities
  • copy back in entity client numbers for all connected clients, force them to retransmit from baseline (lastframe = -1) and directly call ClientConnect to set up their game state properly and make sure they take up the right slots
  • read game + level save game
  • set all spawned clients states to connected; I don't remember what the reasoning for this was, but I think the idea was to keep distance between when the restart happens and when clients are actually spawned back in the game
  • send svc_level_restart message via multicast, which informs the clients that a soft restart has happened and that they should clear all state as if they had just entered the game for the first time. This message in Kex also includes a naive list of configstrings that differ from the current server configstrings.
  • a timer is set to 2 frames; after 2 frames (in SV_Frame) all clients 'connecting' are set to cs_spawned and run ClientBegin directly.