Another attempt at Radicale 3 support

[pvagner]

I combined master branch with #13 PR by exposing missing configuration options and reverting ldap specific logic. My main concern was that anonymous bind has stopped working with the rewrite in #13 .

[nebulade]

Thanks a lot, will try to see if those changes work for me as well. We can then I guess close my PR in favor of this?

[bgstack15]

I added a pull request to pvagner's fork to add SELinux considerations and the multi-server logic from holczert.