Adjust post-install config to handle upgrades better, and a test for debian systems.
- Our default config file has an invalid line to flag that the file has never been touched. It's removed on the first post-install config.
- We guard against overwriting things the next times post-install config is run, like the keystore files.
- There's a "media" setting in the install4j configuration to treat "never override" as config files, which avoid overwriting the config (finally found it), at least on debian.
- Added a new test suite just to ensure the configuration file is not overwritten, only works for .debs at the moment