Player prediction camera lerping and fixes

Fixed unprediction from clearing player inventory selection.
Fixed disconnection leaving bad player mobjs in sector lists.
Removed PacketServer auto selection due to numerous complaints and performance issues in WAN games.
Fixed gateways closing the UDP socket due to inactivity, preventing guests receiving handshake ACKs.

Added player prediction lerping to clear up sudden snaps in movement.
Added cl_predict_lerpscale and cl_predict_lerpthreshold (and menudef sliders in Network Options).