JPackages: No built-by

Built-by on /logs shows "undefined", LogsHelper.getBuiltBy() is looking in (base dir)/lib/i2p.jar but the jars are elsewhere for jpackage installs.

Mac and Windows jpackages need a common way (e.g. System property) to indicate that it's a jpackage install, and/or specify where the jars are, then modify getBuiltBy() to check and look there.

Low priority. Also affects the /jars listing as implemented in FileDumpHelper but that's a debug function and even lower priority.