Dedicated bonuses fix
Fixes the issue reported here: https://mb.srb2.org/showthread.php?t=42530
What it turns out is going on is that making the game bail out in the middle of Y_StartIntermission if you're a dedicated server's host prevents the game from awarding the players bonuses in coop mode. Therefore it's not just special stage bonuses, but ANY kind of bonuses that can cause desyncs if someone loses all their lives.
This can be merged to master since it's a change only for dedicated server hosts, and should otherwise be compatible with 2.1.18.