Ping-related code fix

Somehow, the part of the netcode for calculating the players' pings in a netgame frequently gets the concepts of nodes and players mixed up, which is probably not a good thing. This branch of course fixes those slipups.

I originally based this branch on master to be merged to it (since it only fixed issues on the host's side), but after finding another issue with clients receiving PT_PING from the server, I decided to make this a merge to next instead.