Title: #570: p11_child: add OpenSSL support
Let me review the whole lot and sorry PR #553 was taking too long, but downstream
first, I'll just add comments about the patches as I read them, testing will be next.
So feel free to reply to comments, but let's respin the patches if needed only
See the full comment at https://github.com/SSSD/sssd/pull/570#issuecomment-388336862