Netcode hotfix

Various netcode fixes are included in this branch. Most important of all of course is a fix for the Net_CloseConnection invalid node -32769 detected message thing, or rather what actually caused that to occur.

It's a merge to master since everything here should be compatible with 2.1.18: the only main changes are related to recieving packets (whether you're the host of a netgame or a client), which AFAIK shouldn't cause desyncs if you played with these fixes on a 2.1.18-hosted netgame (or hosted a netgame with the fixes and 2.1.18-using players join you).