Secure delegation or "Mini-DANE"
As with the existing DANE support, only usable in one direction, client
certificate authentication will fail if this is relied on.
For this, various accessor functions are now provided directly on the
sockets, which reach down into the LuaSec implementation to obtain the
information.
While this may seem of little gain at first, it hides the implementation
detail of the LuaSec+LuaSocket combination that the actual socket and
the TLS layer are separate objects.
The net gain here is that an alternative implementation does not have to
emulate that specific implementation detail and "only" has to expose
LuaSec-compatible data structures on the new functions.
