sss_cache: Do nothing if /var is read-only
    
    As suggested in commit ff8307df sss_cache should fail silently
    if running under systemd in offline mode. I suggest to check also
    whether DB_PATH is actually writeable or not.
    
    Also clarify in the comment that we are talking about SYSTEMD
    offline mode, not SSSD being offline.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>