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>