r15400: Move the TLS code behind the socket interface.

r15400: Move the TLS code behind the socket interface.

This reduces caller complexity, because the TLS code is now called
just like any other socket.  (A new socket context is returned by the
tls_init_server and tls_init_client routines).

When TLS is not available, the original socket is returned.

Andrew Bartlett
(This used to be commit 09b2f30dfa7a640f5187b4933204e9680be61497)