Add support for SSH-agent via pipes or TCP

[cipriancraciun]

At the moment the SSH-agent code (mainly zt-exchange tool) allows the agent to be contacted only via a UNIX domain socket. However, it might also be useful to add support for:

• explicit TCP connection socket; (TLS would be a plus;)
• explicit file-descriptor for both ways communication; (any kind of socket connection;)
• explicit file-descriptor-pairs, one for reading, one for writing; (like for example when a sub-process is used to provide the access to the agent;)
• explicit command to be run (that uses the previously mentioned option);