Improved support for Dark Mod standalone

- Added new .game file for the Dark Mod, which does not use a "/base/"
  subdirectory under the main engine path. This should remove the need for
  setting the root directory as the engine path.
- Modified Linux code to only look in a home-directory engine path if the
  "prefix" game file property is non-empty. Dark Mod maps are now saved into
  the main installation maps directory on Linux.