"addfile" console command netgame fixes

See https://mb.srb2.org/showthread.php?t=43144 and https://mb.srb2.org/showthread.php?t=43048

Basically this fix means in netgames addfile now checks properly if there are already too many wad files loaded (whether exceeding the available space left in the filesneeded array, or the hardcoded file maximum), or the file has already been uploaded. Single Player/anywhere else outside of multiplayer already has such checks via W_LoadWadFile, so they weren't a problem there.

This is a merge request to master since only the server should be affected by this fix.