Refactor global variables out of doomcom

The second to last step to get doomcom out of the global namespace, which will greatly reduce the maintenance burden of the netcode. This refactors the global variables that has been nested into doomcom out of it, allowing us to later get rid of doomcom entirely from the global namespace and be able to use it locally instead. This way, we can guarantee that doomcom is always safe to use if we can access it, and it can even make doomcom thread-safe, so we can parallelize the networking code later if we want to (after more refactoring, since we need to get rid of more globals to be able to do this safely).