Add the ability to create Unix-domain sockets

[jamesnvc]

As mentioned on the Discourse.

This seems to be working for me now, although I'm not sure why the pldoc comments aren't showing up.

Tested by starting a socket with nc -lU /tmp/foo.sock in a terminal, then running

?-  unix_socket(Sock),
     unix_connect(Sock, "/tmp/foo.sock"),
     setup_call_cleanup(
         tcp_open_socket(Sock, Stream),
         format(Stream, "hello!~n", []),
          close(Stream)
       ).

[JanWielemaker]

This pull request has been mentioned on SWI-Prolog. There might be relevant details there:

https://swi-prolog.discourse.group/t/unix-sockets-with-swi/2123/4

[jamesnvc]

Test usage now looks like:

?-  unix_domain_socket(Sock),
     tcp_connect(Sock, "/tmp/foo.sock"),
     setup_call_cleanup(
         tcp_open_socket(Sock, Stream),
         format(Stream, "hello!~n", []),
          close(Stream)
       ).

[JanWielemaker]

Thanks. Merged after squash. Added another patch to deal with the server side and which reorganizes things a bit to reuse more code.

[jamesnvc]

Thanks Jan!