selinux: Do not fail if SELinux is not managed
    
    Previously we failed if semanage_is_managed returned 0 or -1 (not
    managed or error). With this patch we only fail in case of error and
    continue normally if selinux is not managed by libsemanage at all.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3297
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit 78a08d30b5fbf6e1e3b589e0cf67022e0c1faa33)