Change label "Not Known" to "unknown"
Change label "Not Known" to "unknown"

In English, I think that the word unknown is being used more often
to indicate that the status of something is not known. Thus I have
changed the label to reflect that.

DoFabien/OsmGo,DoFabien/OsmGo,DoFabien/OsmGo,DoFabien/OsmGo,DoFabien/OsmGo