General TLS library for Proton C

The current TLS functionality for Proton (see "c/include/proton/ssl.h") is tightly coupled to AMQP connections and does not allow TLS sessions for arbitrary content including Proton raw connections.

A more generalized API is proposed that works with arrays of pn_raw_buffer_t content.  As it matures it could serve as the TLS engine for AMQP connections as well.