servfil can generate invalid txt data files when combining local and remote data

[Nielk1]

servfil downloads the latest mater list and other files from this repository and combines the data with local files on disk prefixed with local-. This works for simple additions, but has the following issues:

1. If an item is listed in names.txt and local-names.txt, it will occur twice in the final file, resulting in duplicated lobbies in game.
2. If the local-* file is empty and empty line will be added to the final file. This will cause anet3srv to crash on startup.