NS_USE_EXTERNAL_MBED_TLS now controls whether we attempt to include
mbedTLS header files at all, and after including them, we check whether
SSL/TLS is enabled. If not, we provide non-secure operation only.
Make connection handler deal with all addressing, and hide the internals
of security handler.
Will allow security handler code to be stubbed out if mbed TLS is not
available.
When receiving data, destination (own) address is stored and given to
socket when sending reply to make sure respone comes always from same
address where request was sent. This commit supports only real socket to
real socket messages, not virtual socket, or DTLS handshake messages.