Use separate interfaces for TLS management and TLS state inspection

[Bob Weinand]

shutdownTLS and setupTLS indicate that a sockets TLS can be setup or shutdown.

However, an encrypted socket may want to expose that information without offering to change the TLS state - use cases are when wrapping another socket or also concretely, QUIC, which is forcibly using TLS, setup and shutdown not possible.

[Niklas Keller]

Introducing even more interfaces doesn't really make sense here, so we did the opposite in #100.