Test TLS interoperability between LibreSSL and OpenSSL.

Implement simple SSL client and server in C.  Create six binaries
by linking them with LibreSSL or OpenSSL 1.0.2 or OpenSSL 1.1.  This
way API compatibility is tested.

To self test each SSL library, connect client with server.  Check
that the highest available TLS version is selected.  LibreSSL TLS
1.3 check has to be enabled when the feature becomes available.

Currently OpenSSL 1.0.2p and OpenSSL 1.1.1 from ports are used.  As
soon as LibreSSL supports TLS 1.3, it should be used automatically
when netcat is communicating with OpenSSL 1.1.

Connect and accept with netcat to test protocol compatibility with
libtls.  Test TLS session reuse multiple times with different library
combinations.  The cert subdir is testing all combinations of
certificate validation.  Having the three libraries, client and
server certificates, missing or invalid CA or certificates, and
enforcing peer certificate results in 1944 test cases.  The cipher
test establishes connections between implementations for each
supported cipher.