WIP: PlayerQuit Improvements

The PlayerQuit hook now works for the non-dedicated server along with singleplayer and splitscreen sessions. Also, the PlayerQuit hook executes in Got_KickCmd instead of CL_RemovePlayer when a player leaves the server, except when the server shuts down. Otherwise, the PlayerQuit hook executes inside of D_RunPreQuitChecks. Please make sure MR !964 (closed) is merged first, before this MR does get merged if this MR does get approved. A new test EXE will be uploaded after I finish improving the PlayerQuit hook.