Right now only a global tls directive can be specified.
Allow to nest a tls directive in each frontend to customize per-frontend tls options (depends on #8).
Handling ACME options is tricky when we have multiple frontends sharing the same domain name:
frontend example.org:4242 { tls { acme_ca http://a } } frontend example.org:4243 { tls { acme_ca http://b } }
I wonder if we can do better than just reject conflicting options.