Added support for unix sockets

[kSkip]

I'm working on a Fast CGI server for my ML models and would like to integrate supporting changes into dlib's networking module. One of these changes is supporting socket type AF_UNIX, so web servers like nginx can bypass the network stack when communicating with the fcgi server. Another is a listener factory method that can accept connections on a user-provided socket. In many fast cgi setups, the application is expected to listen on stdin (which is bounded to a socket), so the user-provided socket is helpful here.

I implemented the required changes for both posix and win32 including tests. However, I'm not sure how to test the new create_listener_from_socket method. It would require platform specific code in the unit test.

[davisking]

Suggested change

	- sock is either a ICP/IP socket or a Unix socket
	- sock is either a TCP/IP socket or a Unix socket

[davisking]

I implemented the required changes for both posix and win32 including tests. However, I'm not sure how to test the new create_listener_from_socket method. It would require platform specific code in the unit test.

How are you making the socket you have when you call create_listener_from_socket() in your real application? Can't just do basically that in the tests or even better have an abstracted generic method for doing it?