libticables: implementation of the TCP cables

[debrouxl]

After being suggested years ago, the TCP client and TCP server cables are now part of libticables' API, but they aren't implemented yet. One of the ways not to reinvent the socket handling wheels, and have to deal with all kinds of functionality (send with timeout, etc.) and portability issues ourselves, is to take advantage of Glib's sockets, or ZeroMQ ZMQ_STREAM sockets. Using raw socket APIs would be more suitable for Emscripten builds, though.