Add "Prerelease" notation to PRETTY_NAME and VERSION in os-release
    
    If the release field of fedora-release starts with 0, it is a
    pre-release. Including this in PRETTY_NAME and VERSION will make it
    easier to identify in other applications whether this is a stable
    or in-development release.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>