Small changes to archives

Changes to archive scripts:

* Renamed archive methods and variabled from `nlxtp_...` to `archive_...`
  - Also kept `nlxtp_dir` for backward compatibility
* Passing on `archive_setstatus` via environment instead of eval string
* Changed version function to a version variable. This makes it possible to use the version number in the script. Users can still set the status to the version string by `archive_setstatus $archive_version`
* Bumped up version to 0.3 to match tomato major version