bring back OS distro+version into features, unset device variables (features,version,distro,serial,etc) when their contents disappear

git-svn-id: http://observium.org/svn/observer/trunk@4472 61d68cd4-352d-0410-923a-c4978735b2b8
don't reset icon

git-svn-id: http://observium.org/svn/observer/trunk@4473 61d68cd4-352d-0410-923a-c4978735b2b8