Improved "respawn where died" functionality (#2831)

Now uses a "last safe position" system instead. This:
-Prevents players from slipping into places they shouldn't via their dead body and then respawning there
-No longer sends players back to spawn if they fall into kill pits
-Adds a safety feature where the kill command always resets back to spawn should a player ever get stuck somewhere

src/g_game.cpp

@@ -1438,6 +1438,7 @@ void FLevelLocals::PlayerReborn (int player)
 	p->cheats |= chasecam;
 	p->Bot = Bot;			//Added by MC:
 	p->settings_controller = settings_controller;
+	p->LastSafePos = p->mo->Pos();
 
 	p->oldbuttons = ~0, p->attackdown = true; p->usedown = true;	// don't do anything immediately
 	p->original_oldbuttons = ~0;