Fixed: Large frag messages still appeared on the screen for spawn telefrags in online games.

In online games, when the spied player frags another player, or gets fragged by them, a large frag message still appeared when the means of death was SpawnTelefrag when it shouldn't. This is because clients could execute HUD_PrepareToDrawFragMessage in AActor::Die, despite MeansOfDeath (a global variable) being uninitialized on their end. It's better if this function only triggers in ServerCommands::KillPlayer::Execute where the MOD is sent with the command.